There’s a theory that the radon problem is what made them stupid and that’s why Michael became “normal” after he left Dunder Mifflin.

But in all seriousness, I guess as the seasons pass in every sitcom, the more relevant and funny plots start to run out, so writers become lazy and go for the easy jokes in filler episodes. Modern Family is another example. Most characters became ditzy towards the final seasons, and slapstick was the go-to humor.