They could do that, but then would need to also immediately require a big [X] next to "the instant you do this you forfeit any and all support for any hardware or software issues on your device".
Apple doesn't want to deal with a million hackers screwing with their devices and clogging up their support system, and also then having to deal with endless reports of terrible customer service because someone paid 1000$ for an iPhone and then Apple won't help them out when they brick it with the latest flashware they downloaded from usenet. Followed by the inevitable complaints that "Apple Customer Care is terrible". Easier just to control the entire ecosystem, and tell folks who don't like it, "cool, cya".