Because it's not just a 'toon about pizza delivery girls with big knockers delivering pizza topless. It's about the sexual objectification of women and how they sexual objectify themselves to garner attention from the less sophisticated of the male species that rank physical traits higher than intelligence when seeking compatible sexual partnerships.
And there is also self reference that puts the cartoon in the fold of reality where things overlap and bleed into the other. "Fred 'n' Peckerhead" are a cartoon about metal heads on the surface, but there is a much deeper level that explores the truths about reality. Now I've manipulated you in wondering if I smoke pot.That's because I'm a MASTER of the craft.