That doesn't allow you to do a large number of things that ordinary apps can do -- or that "approved" apps also can't do, e.g. by modifying parts of the system in advantageous ways.

You can also obviously just buy a PC and run whatever you want, but the entire point is the barrier to entry. If it's easy, more people do it, and that's good. If they have to contend with a convoluted mess, people give up, and you lose whatever they might have created.