Dilbert doesn't quit because almost all of the mini-storylines contributed are from real workers (to Scott Adams) and he acts as silent mouthpiece for us anonymous workers
Yes, and while I could tell him quite a few tales about dysfunctional startups, there just aren't as many such stories in the world. The number of employee hours spent working for dysfunctional startups is necessarily limited, because such companies evaporate faster than the morning dew.
I also wouldn't be surprised if, to paraphrase Tolstoy, broken startups tend to be all alike, while each pointy-haired boss in the depths of an enormous company hierarchy is broken in his own way. There's a lot of ripe dramatic possibilities in the artificial environment of a BigCo; that's why so many dramas (and even musicals!) have been set in offices.
But for all I know the "Dilbert-does-a-startup" storyline has actually happened several times. I wouldn't expect such a storyline to last much more than a week. There's only so much humor you can get out of a company that's plummeting toward the ground, trying to learn to fly by delegating an intern to flap his arms.