Because there are lots of things a computer can do comfortably that a phone, tablet or chromebook can't.
Sometimes there can be tradeoffs between convenience and security, but in the case of an app store the convenience actually enhances the security, because it diminishes the chance of people running random harmful commands from the Web.
There are a lot of things a metal lathe can do that other tools can't. But that doesn't mean a lathe is a good tool for everybody. In fact, it couldn't possibly be, because there are trade-offs between usefulness and safety. You just can't make a lathe that's safe for the casual user but useful to a professional user.
