To reach Android users, you must go through the Play Store.

Realistically, these stores are not in competition. They have monopolies over their respective markets. If one store offers a better deal to developers, that doesn't meant the developers can all flock to that store and abandon the other one. Developers still must deal with both. As a result, the regular forces of competition aren't doing their job here. Neither store has any incentive to make their terms better because they won't gain any developers by doing so.