The throughput is arbitrarialy limited by bitcoin's current block size, which hasn't been increased since satoshi's era.
Most cryptocurrencies have an adaptive block-size mechanism which allows the blocks to grow to a reasonable size which could facilitate such an onboarding of users. So it isn't a technical problem, it is just a question of bitcoin's current leadership, which is controlled by companies like blockstream.
People have been debating the blocksize for a very long time now and there doesn't seem to be any large desire to change it so while the ability to increase it exists changing anything that fundamental about bitcoin seems to be a non-starter and while that is true lightning is pointless as a solution for the masses.
Even if you increase the block size 100x though you're still not improving the numbers much since my very generous numbers ignore activity outside of lightning and assume a single on chain transaction for every user and a perfect network.
Maybe your understanding of lightning is wrong here. Yes you open channels, and transactions in lightning need open channels, but you do NOT NEED to open channels for specific transactions. You open channels once, and transact over them for years. I run a lightning node with more than 15 channels (each to different lightning nodes) that are all older than 1 year (I route payments, so I have way more channels than needed). You can batch-open channels, i.e. I could have opened all my 15 channels with a single on-chain transaction. Taproot update would make those "commitment transactions" onchain way smaller (in byte) than needed to in the past.
Once channels are open, the users on the lightning network can transact back and forth without any new channel opens/closure and thus no on-chain settlement. Hence: Throughput in lightning is not at all limited on the bitcoin transaction throughput.
Most cryptocurrencies have an adaptive block-size mechanism which allows the blocks to grow to a reasonable size which could facilitate such an onboarding of users. So it isn't a technical problem, it is just a question of bitcoin's current leadership, which is controlled by companies like blockstream.