Incompetence in the management mostly. Because if management were competent - incompetent devs would've improved their competence or got fired.
And here's the kicker - Game Dev Industry is as bad as any other industry. It is a business like all the others. Run mostly by simple businessmen who are bad managers and hire bad managers. The bigger the company is the more incompetence there is within its ranks. Hence why AAA is so expensive, gets delayed, and still has bugs.
Just like there are very few decent companies in other industries - the same is true for Game Dev.