Steam charges a one-time $100 fee per game, so it’s still similar if upfront publishing costs are the issue. But certainly that $100 is less worth it if there are hardly any Mac players and thousands of Windows/Steam players.
There's a big difference: The Steam fee "will be recoupable in the payment made after your product has at least $1,000 USD Adjusted Gross Revenue for Steam Store and in-app purchases." https://store.steampowered.com/sub/163632
a hundred dollars. A year, thats 0.27 cents a day. The boomers tell me that's just 5 avacado toasts a year.
Its quite a lot to pay just to make a gamejam game available though, and that is where indie developers starts out. If you can't publish your toys without paying 100 dollars then it is a huge issue for onboarding new developers who just want to test what it is like.
The $100/yr is to publish to the App Store, which I don't think is the place for a gamejam game. You can download Xcode for free and distribute the resulting binaries for free.