Realistically most people (numerically) don't want a computer, they want a thing that lets them communicate, create and consume content.
Lockdown isn't a problem for them, and walled gardens like iOS are generally the best way for them to stay secure.
If you provide a method to escape the sandbox, it is likely that attackers will work out how to exploit that. For example try going to facebook.com and open the developers tools. They've had to put a warning in there to stop people from "self-XSS" ...