Apps being able to overlay stuff onto other apps is a double-edge sword. You get good uses of it like f.lux and things like 'friend is now online' notifications with steam, but then you also get arguably user-degrading experiences like adware and the old Facebook messenger chat heads[0]. Apple and others made the decision to not allow this at all, so the only apps that can survive are those that can leverage notifications that users can always turn off in an uniform way.

0: https://images.app.goo.gl/Kzf3S8vNL9zB28cV6