As far as I know, it's because they do not have partners.
Chromium is competing with Samsung internet and others on Android, because several companies sell Android.
So it's not fine for Google to force the choice.
Apple is not competing with anyone else on iOS, you don't have a choice.
As long as iOS does not have a monopoly in smartphones, they're in the clear.
Doesn't that seem a bit backwards? Google faces anticompetitive scrutiny because they created an open(ish) platform that others can compete on. But Apple goes all in on a walled garden where nobody even has the opportunity to compete with them. Doesn't that feel anticompetitive?
I'm certainly no friend of Google and I'm not losing sleep over them getting fined. But it seems that Apple is just as much if not more anti competitive and anti consumer, but they get a pass because of what feels like a loophole.
At the same time, it makes kinda sense. No company with a product has to allow competition in. GM doesn't have to allow BMW motors in their cars.
When you join a market with a product, like Apple, then you don't have to allow competition in your product.
When you create a market like Google did Android, then you to allow competition in that market.
I think that's the key difference, product vs market.
Chromium is competing with Samsung internet and others on Android, because several companies sell Android. So it's not fine for Google to force the choice.
Apple is not competing with anyone else on iOS, you don't have a choice. As long as iOS does not have a monopoly in smartphones, they're in the clear.