it's because the demographic you're talking about is vanishingly small, and therefore apple does not care about it. do you notice them hurting for customers?

i installed linux when i was 12. i've been working in this field for nearly 20 years. i'm abnormal, as 'techie' as they come, and i really just don't give a shit about having root access on my iphone. in fact i pretty much use it for a handful of things, primarily messaging, and that's it. i view my phone as a fancy microwave oven.

from a business point of view, you might as well ask amazon for root access on their hypervisors, or network switches. some people give you that. others don't, because they just care about the people who want that.