Because google lets you sideload. Even though almost no one actually does this, the option still exists in theory so you aren't forced to take googles offer.
Would I prefer if there was a truly FOSS mobile operating system? Yes. But life is what it is and I'll take the /option/ to use F-Droid over only having the App Store any day.
Maybe its because 30% is a relatively small price to pay for the benefits of app stores versus trying to get users to manually install your app via side loading.
Asking Apple to allow side loading and alternative app stores is like forcing Tesla to make a gasoline powered car.