This is one of my few problems with the show which they kind of talk about in the podcast.

In the beginning, everybody was about on the same playng field. Sure Dennis has a college degree, and Dee has a little bit of college herself but at the beginning they all do stupid things and all trick each other.

My complaint is that the characters become parodies of their most extreme behaviors later. They play up Charlie's inability to read and write, habit of eating weird stuff, etc so much that he becomes the stupid one. Frank, who originally was the smartest character to me, becomes an old man with dementia. Mac is the worst example of this as he goes from easily tricking Dennis into having a threesome with 2 dudes to completely oblivious.

I suppose you could say this is part of their character evolution. In Charlie's case of course is the episode Flowers for Charlie where sure charlie is shown to be a doofus in the end with his talking animal experiment, but is actually smarter throughout the episode. Evidence of this to me is his knowledge of the placebo effect which he clearly understands at the beginning of the episode but not at the end because his friends drag him back down (where foundations go).

In any case, in the podcast they say that in the beginning the characters were believable in the sense that yes they were terrible people but they could have existed. You could have met these people in the real world but now they all live in their own sort of reality. Despite my complaint, I honestly don't see how they could have done things differently considering how long the show has been on. You always need to come up with new material and that becomes harder and harder the longer you run.