It’s very simple: support. Apple does not want to take any more needless support calls than they have to. When you run a trillion dollar company that handles, I would speculate, tens or hundreds of thousands of active support tickets, and consider adding in a virtually unsupportable mode in your appliance (keeping in mind that iPhones are considered an appliance to Apple), I would hope you would choose the sane option.

Also, Apple prides itself on quality. Side loading, whether you agree with allowing it or not, degrades quality because Apple loses its end to end control. See Android phones as an example of this.

All that being said, sure I would love to have the option of side loading, but if I really care that much, I’ll jailbreak.