Well, one carrot for Spain was the assistance it received during the last financial crisis[0], and France probably likes being able to project its influence onto neighbouring countries through its position in the EU.
Less reductively, I think that a major benefit of a political union is that it gives some degree of democratic legitimacy to decisions made about difficult region-wide issues like trade policy, consumer protection, immigration, the environment, and anti-trust law, to name but a few.
All of those matters could in theory be settled via multi-lateral treaties, but they would probably be hard to reach flexible compromises over while still being overseen by representatives voted for by the public with a mandate for deciding on those supranational issues.
Why bother with barely a vaneer of legitimacy at the EU level which has a democratic deficit?
Why not have an EEC that casts flexible treaties over immigration, environment and anti-trust on as as-needed basis, with actual elected represented 'regular' national MPs dedicated to those issues at the European level? i.e. Boris would have 5 Cabinet Ministers for major European level issues and a European ministry?
That way have the benefits of some degree of coherence without the sovereignty problem.
So your solution to the "democratic deficit" of the EU is to give Boris more power? He already has, in practice, all the power of the unelected head of state at his disposal, plus the unelected House of Lords, based on a voting system which put him into power with 44% of the popular vote.
"He already has, in practice, all the power of the unelected head of state"
B.J. is an Elected Head of state. He won his mandate in a very clear and unambiguous election, the primary issues of which concerned not only on his leadership and tenure, but also the pivotal issue of Brexit. The election had very high participation rate, and the UK public was very well informed with respect to the stakes involved. B.J. doesn't have absolute power and both his party and opponent MPs can challenge his authority.
The EU meanwhile, has barely any democratic legitimacy.
The President of the Commission and members of the Cabinet - those that enact legislation - are unelected.
Ursula Von Der Leyen did not stand for election. She was plucked from obscurity by Macron and Merkel after the EU elections, in a closed, backroom deal in which voters had no effective influence. The protocols by which she was chosen are not codified, in fact, it's only recently agreed to (Treaty of Lisbon) elected MEPs should even have to be consulted.
Von Der Leyen was not introduced to the public before the election, she didn't go on a media circuit so as to be introduced to the public of various countries where people could learn about her or her platform. There were no editorials, there was no media vetting, there were no interviews, there was no pragmatic by which voters could even learn about their would be leader. She was imposed in EU voters, probably less than 2% of whom had even heard of her.
EU elections have very low rates of participation, EU voters have very low awareness of any of the material issues, MEPs have very little ability to influence legislation and very limited ability to censure leaders.
So yes, Boris Johnson (and Macron, and Merkel) have considerably more legitimacy than any members of the EU apparatus, and that might very well serve as a better basis for European citizens to influence the establishment of rules or laws via treaties as opposed to the very indirect democratic mechanisms of the EU.
Just so we're using the same terms, here, I think it is conventional to describe Johnson's role as "head of government".[0]
> members of the Cabinet - those that enact legislation - are unelected.
The Commissioners are appointed by the heads of government of the member states, exactly the process you suggested in your earlier comment. If you think that Boris would be limited to picking ministers who had actually gone to the trouble of winning an election, then you'll be surprised to read about the 19 unelected ministers that Boris has chosen.[1]
> Ursula Von Der Leyen did not stand for election.
While Boris did manage to win 25,351 votes in his constituency, and 92,153 in his leadership election, there is nothing preventing a UK prime minister from gaining power through a backroom deal, such as Theresa May's unopposed leadership "election" in 2016, or Brown taking over from Blair in 2007.
This is of course accepted since the PM must hold the confidence of the parliament, but the same is true of the Commission President, who must receive the approval of an absolute majority of MEPs.[2] In fact she (together with her Commission) was elected with 461 votes to 157.[3]
> She was imposed [on] EU voters, probably less than 2% of whom had even heard of her.
I agree that this is a failing of the process and the political culture of the EU.
> EU elections have very low rates of participation, EU voters have very low awareness of any of the material issues
That could be interpreted as meaning that a large proportion of EU citizens don't feel particularly affected by the EU's decisions, which would mean that the EU is (correctly) leaving national issues to national politicians (who then influence the decisions of the Commission through their appointed commissioner).
> MEPs have very little ability to influence legislation
Any influence the MEPs lack just means more power to the national governments who control their respective commissioners. I don't see how this process is any worse than having an unelected House of Lords which contains hereditary peers.
> and very limited ability to censure leaders.
MEPs have the power to call a Vote Of No Confidence in a Commission, which can lead to its dismissal. This came close to happening in 1997 during the BSE crisis.[4]
> So yes, Boris Johnson (and Macron, and Merkel) have considerably more legitimacy than any members of the EU apparatus
If by "any members of the EU apparatus" you mean "specifically Ursula Von Der Leyen" then you almost have a point. I would also be more convinced if you had limited the first part of your statement to just Merkel, as I believe that Germany's electoral system is more proportional than the UK's and doesn't have the "safe seats" problem of FPTP. (The two-rounds system that Macron was elected under has its own problems).
As a thought experiment, imagine that Biden had won the presidency with just 44% of the popular vote, and that win also granted him an effective super-majority in both houses of Congress (since the UK only requires a simple majority to force through constitutional changes). I can only imagine that scenario leading to civil war in the US, and yet this equivalent situation in the UK lets some people think they can throw stones at the EU's democratic character.
Less reductively, I think that a major benefit of a political union is that it gives some degree of democratic legitimacy to decisions made about difficult region-wide issues like trade policy, consumer protection, immigration, the environment, and anti-trust law, to name but a few.
All of those matters could in theory be settled via multi-lateral treaties, but they would probably be hard to reach flexible compromises over while still being overseen by representatives voted for by the public with a mandate for deciding on those supranational issues.
[0] https://en.wikipedia.org/wiki/European_debt_crisis#Spain