It's a rare employee who doesn't think he can do a better job than the manager. If he does start his own company, he'll discover his employees are sure they can do better than him.
It’s an eye opening experience making the jump from one to the other; you realise that often the technical nitty-gritty is a small fraction of the overall business, and that the obviously technically correct decision can simultaneously be a terrible business decision.
And it is very rare when they actually can, most times they try or are given the chance to it fails. Almost nothing is as easy as it seems from the outside, and giving a critique is a lot easier than executing.
It's an immutable fact of life.