Why were there so many hallucination episodes?

[neatoman]

So I'm rewatching season 4 and I just got through Journey to the Center of Mikey's Mind. That episode got me thinking, why were there so many episodes featuring hallucinations or illusions?

Fungus Humungous, Mazes & Mutants, In Dreams, Vision Quest, The Deadly Venom and Journey to the Center of Mikey's Mind all featured the turtles dealing with illusions or something similar. Giving us an average of one per season.