A handy person could probably do the brakes. He isn't going to true the wheels properly. It might be straight, but it may have too much tension and cause a failure at the hub later. If the stem is not inserted at the proper height it may break off. Joe garage mechanic doesn't always know these things.
I do my own bike maintenance (except building or truing my $500 rims), so yeah it doesn't take a 4 year degree. But people aren't as handy in general as they once were, and bikes are more complicated.
It's just a view held by many avid bikers such as myself. I'm not declaring any new law be made.