Because the entire definition of "open" and "closed" platforms is an arbitrary one dictated by the platform creator. There is no reason that iOS (and every game console for that matter) can't run arbitrary programs provided by the person who bought and paid for the device other than obstinance in service of rent seeking.

Why should "closed" platforms (i.e. ones where the user doesn't get to decide) even be allowed to exist?

I'm not comparing anything, I'm saying closed platforms shouldn't exist. the practical difference is however that the mobile phone is used as a general purpose computer and is at this point for many people the only device they actually use. That said I am not in principle opposed to apply the same standard to gaming consoles, but it is less of a pressing issue compared to the fact that there are hundreds of millions of people with smartphones who basically have no access to any remotely free and open computing environment.