Why not allow other competitors to take over these software responsibilities if they want to? Why can't Google Play be on iOS too? Do you think Google wouldn't be able to effectively curate, distribute, and secure its own app store?

Yes, Apple builds excellent hardware, and deserves to be rewarded for it. They are rewarded when I give them money for their products. They shouldn't have a say in what I do with the product after I receive it. Imagine buying a new Ford or Nissan and the dealership sets which highways you're allowed to drive on!