iOS is their own product. Every company has its own products on an "iron grip", as well as controlling who gets to partner with them (and e.g. sell something on their platform).
That's because Microsoft allows it, and (b) Microsoft has a monopoly on their market (not on the "Windows market", but like 98% on the whole Home/Enterprise PC space) and abused it so they were forced to open up.
If Microsoft had just a small share of the Home/Enterprise market with Windows, they'd could close it in any way they want, same way the game companies do for their consoles or Apple for iOS.
I think the answer is much simpler: at the time Windows was created, no one in their most sadistic totalitarian dream could have imagined such evil as an app store.
iOS is their own product. Every company has its own products on an "iron grip", as well as controlling who gets to partner with them (and e.g. sell something on their platform).