The simple social rule that explains this is pecking order. One of the most basic human desires is dominance over others. When you have the money, you hire as many submissive minions as you can. "Bullshit jobs" are just an entourage. Then you display dominance over them by making them do meaningless work - the more humiliating the better. If you hired people that were actually useful, you'd be reliant on them so only get as many as you actually need to keep your organization from going under.