A big reason is the backup camera requirement[0]. Why just have a screen for the backup camera when it's marginal to add on the other features.
Still seems like it would be better to just include what's required and then let a phone handle the software.
My 2015 piece of junk requires a $500 p.a. subscription to receive traffic data for the navigation system. That is about 5 new Garmins a year, or 1.25 Pixel 3a's, a phone I don't own but have heard good things about.