Because there are already users with Apple devices and those users might have been locked into using Apple's ecosystem for other reasons (already purchased apps, already owning Apple accessories that don't work with other devices, etc).

As an anecdote myself, the main reason I haven't switched to a Galaxy S24 is because my Airpods work amazingly with my iPhone and Macbook, and my Apple Watch only works with iPhones. But very often I sorely miss having Termux, NewPipe, Tachiyomi, a non-gimped version of GBoard, Syncthing, a sensible launcher, and probably other things that I can't remember off the top of my head. I've decided that I value the Apple system more than the value I get from those apps but this regulation means I get to have my cake and eat it too.

Because Apple is not necessarily entitled to an anticompetitive market just to provide product differentiation.