Why do you think those particular companies deserve a larger share of good employees.
Boeing hasn't been innovative in a while now. They make most of their money off of sales of their older models and defense contracts.
GM isn't doing so well these days and has moved most of their manufacturing overseas. Not exactly a win for the American economy. Consumer automobiles isn't exactly a growth industry right now.
Tesla and SpaceX are promising, but still small. I don't think they have that much trouble hiring. Due to their reputation and high compensation levels, they can easily compete with Wall Street for the best and brightest engineering minds.
So the question remains, why these particular companies? Are you really just talking about old-school heavy industry companies in general? Why not other sectors, like electronics, medicine, and IT?