That knowledge is an entire career's worth of knowledge. Sure, almost anyone could pick it up, but it's more cost effective for them to rely on others to do it for them.
For a musician, the learning and upkeep is a lot of time that would better be spent creating; making a living.
I don’t disagree. But that’s the tradeoff I’m talking about. Letting others do it for you means giving up control; the limiting case is being abused by YouTube. If you have money, you can hire people to do this for you. But most people can’t afford to hire their own “engineers”. These are the choices.
For a musician, the learning and upkeep is a lot of time that would better be spent creating; making a living.