Aside from the moral clamor, if something has a higher likelihood of fraud, there's a direct relationship with the increase of its cost. Both legal fees and labor cost to deal with these claims could add up more than we outsiders may realize. It's very possible that some risk-averse analyst "ran the numbers", and decided this wasn't worth it. I would also speculate that there may be a certain hidden coat of false fraud claims. Certain folks buying something in the moment, then shamefully claiming they didn't after the fact, which in turn could carry the costs associated with processing a new card & number or conversely fighting false claims.
As for the morality angle though, while I definitely agree that these companies' main motivation has to be increasing revenue and profit, and that their only reason for doing anything is cost-driven; you never know what middle-manager who is swayed by what belief is actually making these decisions. So as much as the monolithic goal of the organization is more money, there are still emotional (and financially fallible) people pulling the levers.
The fraud thing explains why they might avoid an entirely adult storefront for it, but for steam? who has their own refund policies and support system that presumably shields the payment processor from charge backs most of the time?
There are also large anti porn lobbying groups applying pressure to the payment processors, so that angle creates costs in a different way.
Steam’s refund policy and support system doesn’t eliminate the possibility of someone buying on steam with their CC and then calling their CC and claiming fraud.
As for the morality angle though, while I definitely agree that these companies' main motivation has to be increasing revenue and profit, and that their only reason for doing anything is cost-driven; you never know what middle-manager who is swayed by what belief is actually making these decisions. So as much as the monolithic goal of the organization is more money, there are still emotional (and financially fallible) people pulling the levers.