The tricky part with an unjailbroken device (for third-party developers) is that apps can’t fork/exec, or any of the equivalents— aside from a couple exceptions around app extensions, the binary that gets run when the user taps the app on the home screen is the only one that ever can be run.
That’s fine for most GUI-driven applications, but it’s kind of problematic for a traditional *nix-style shell. If you were Apple, or you had a jailbroken device and could circumvent the sandbox, you wouldn’t have any problems running a normal shell and command line utilities.