What you're really asking is why is there no root shell by default on all phones, and simply posing the question also answers it - it's a phone, there's zero demand for messing about with terminal emulators on a phone. If you say, but I'd like to install apps as root, the problem is such apps could break the phone in arbitrary ways like by infecting it with malware/adware/spyware that then can't be uninstalled. Modern operating systems are very much designed to limit what apps can do to avoid the nightmare that desktop OS's turned into, where people are afraid of installing apps in case something goes wrong. For better or worse you can install apps on Android/iOS without much fear, which is one reason people do it so much: you can always remove the app again if it's trouble.
Android is pretty customisable and has lots of permissions anyway. There are very few things you can't do if you don't have a carrier-locked phone. So supporting apps with root access would vastly worsen the malware situation in order to satisfy a tiny number of geeks who like editing config files: a very bad tradeoff.
But hey, Android is open source, so if you think I'm wrong go ahead and make money selling phones with root access.