This is pretty true for a lot of people and organizations. The best engineers almost without fail are the ones who are passionate about it and don't want to be

This is pretty true for a lot of people and organizations. The best engineers almost without fail are the ones who are passionate about it and don't want to be doing anything else. They'll tend to keep being engineers. And corporations tend to recognize, value, and promote managers based on team size and growth as key metrics. Can't blame someone for doing what their employer tells them it wants. The problem here is not caused by line managers but by executives who set goals in this way. And I don't really know what line managers do either. They seem to sell themselves to their reports as "dealing with all the corporate bullshit" which is kind of true, but if the corporation is generating so much bullshit that it needs to employ bullshit wranglers, it seems like the better solution would be fix it at the source rather than clean it up after the fact. I get there will always be some amount of administration and stuff that you don't want your engineers spending lots of time on, but that could be handled with a personal assistant type of role that should be able to handle dozens of engineers per PA, rather than a higher paid manager.