Why can’t one develop apps on a mobile platform?
I suspect this will be considered off-topic for the site, but nevertheless…

As it currently exists, Xcode is completely incompatible with iOS. Presumably at some in the future there will be some way to develop at least with Swift on iOS.

The only real answer is, of course, that no one who knows the answer to this can or will say, so there can be no real answer to this other than "Xcode doesn't run on iOS" which doesn't explain why Apple hasn't made it possible to develop for iOS on iOS before now.