Because for many applications it's not viable to not provide service for iOs users, which is sad but true.
Consider you not being able to:
- buy train tickets under iOs
- check-in into a hotel under iOs
- do video conferences
- chat with your friend
- cross play with your friends
- open your apartment doors
- check your health data
- do online banking
- securely check your email
- do Ueber, Rbnb, etc.
- buy at Amazone
- etc.
Sure many (but not all) things might be doable over web interface but this crates often much more frictions and due to constraints put onto web interfaces and PWS also has often much worse UX, not providing service for millions of iOs users is often deadly and providing competitively shitty service isn't anygood either, even worse if you have to compete with Apple itself which gives there apps unfair advantages beyond not needing to pay the 30% cut.
Let's be honest any marked which covers millions of people should be handled as a standalone marked even if you could argue it's just part of a bigger marked by any government or you fail your people (and the free marked idea IMHO, but reasoning about this is tricky as it involves considering how the marked dynamics have fundamentally changed and how thinks which would have worked in benefit for the population don't work anymore in the current times and therefore how the marked (laws) has to adapt to it).