Backgrounds and ringtones don't require any meaningful comprehension. It makes a lot of sense to assume the user would get an itch to stop looking at the same pictures and hearing the same ringtones all the time. That's as far as their desire for customization would go. It's similar to why cars have stereos and paint colors. Anyone can pay money for more customization, but few do.

Inbox rules are just about the simplest actually useful thing someone might do, but probably doesn't. That's a pretty clear signal to me they don't care. That end user cognition is the actual barrier.

In so many words, the user is dumb and complacent. Computers aren't going to think for them no matter how advanced the AI. They also don't want the computer thinking for them.

They want to get things done with stability. Computers are not their focus in life. Imagine an office full of people constantly changing how they work and forcing and politicking their bullshit onto other people (like devs do all the time). That's a normal person's nightmare.