The hierarchy can get super regimented in a lot of medicine -- university labs, especially hospitals, etc. Think med student / resident / intern / fellow / attendings / chief / etc., and nurses / nurse practitioners running on the side. My wife (md/phd fellow) currently works ~6 days a week, ~7a-7p, and with constant dangers of patients dying, being sued, and over-worked colleagues flipping out. Imagine everyone is on ops: everything is permanently on fire, you spend the morning learning about the latest fires, legal issues surrounding almost every action you take & hand-off, and getting waked up at night to deal with whatever went wrong with the day's plan.
Startups, and especially big tech, are relatively easy. More money etc., but no imminent risk of say death or infection.
That sounds gruelling, but, as long as life is on the line, then it can be much more satisfying than, say, fixing some critical server issue at 3am. It's all a matter of perspective.
I've heard the same from law firm "lifers" that left behind the goal of becoming a partner to join a startup. They thought startup life was a life in the park comparatively.
Even MD's still have to deal with some annoying hierarchy or another, unless they own their own practice.