Yea i don't get it, usually when you work on something, you're slower in the beginning but faster and more efficient in the later portions of the project, because you just get better at it over time! But, when it comes to these games, many devs seem to get slower and slower instead
My thoughts tend toward a few factors for this.
First, dev keeps trying to shake up the formula. Adding zones with stranger and stranger gimmicks that the engine really can't handle all too well. Understandable, but really something that should be done in a sequel or whatever game is made next. Not in the middle of a project.
Burnout, loss of motivation, whatever you wanna call it, it is real. Now, for most endeavors of this scale, there's usually things pushing back and working to make the dev keep the pace up. Unfortunately...
The payment model for this does dev no favors when it comes to wanting to work quickly and keep delivering updates that are both frequent and substantive. This really hits development in two ways. First we got the fact that if goobers are gonna keep shoveling money in regardless of productivity, the monetary angle is gone. Second. If the game is ever completed, there's no telling the audience that's willing to fund the dev will stick around for the next project.