It still is a damn good deal. Steam abstracts
a whole lot of messes. In ye olde times you as a game developer had to acquire a publisher for each country you'd plan on selling your game to deal with local distribution structures and laws, taxes, payments, update distributions, DRM and anti-cheat, user management...
Steam conveniently abstracts all of that for you. One stop shop. No complex deals just to deal with getting paid for your game (or additional content), barely any chargeback fraud, you don't even have to deal with stuff such as Germany's highly complex age rating because Steam abstracts that with a questionnaire. Steam claimed to recognize and support 237 countries [1], although that list includes disputed countries, so take it with a grain of salt, but in general I'd say unless a country is affected by US sanctions (i.e. North Korea, Iran, Russia, Belarus) or has its own restrictions (i.e. China), chances are 99% you as a publisher can sell your game in this country with everything being taken care of.
And on top of that, gamers likely will already have a Steam account with payment already set up, which means far, far less friction than the likes of Epic Games impose.
That definitely is worth a cut.
[1] https://news.ycombinator.com/item?id=40263518