I think this is indeed a problem of awareness - and I'm sure you're right that companies wouldn't go out of their way to let users know they can do so - but also one of trustworthiness: I think a lot of people don't trust third-party repair shops because they feel it's sketchy or unsafe in some way. If phone manufacturers were like, "Here's a list of our trusted third-party repair shops and the parts they can fix" I imagine a lot more people would get their stuff repaired.