Because making the game completable by the game testers is on the checklist of things they need to do as part of development.
Making the game engaging to play is not on the checklist, because it's not something measurable.
I can validate that the most retarded tester ape is able to walk from the start to the end of a level by covering it in pavlovian response causing yellow paint.
I can't validate that a person of average intelligence found the level engaging and interesting.

It's part of the pattern of organizations emphasizing things which are measurable and treating things that are fuzzy as though they don't exist.
It's why game spend so much money on assets, but spend fuck all on game system design. The middle management can validate the assets are made, they can't validate that a game system is more interesting and offers more interesting builds.