...because it's stupid. That's why.
Captain Planet was nothing but a politically motivated PSA, courtesy of one Ted Turner. It was basically all of his views crammed into one awful cartoon that features some of the WORST of everything, whether it be heroes, villians (or any other character in general), jokes (not to mention, joke execution)... just... EVERYTHING about this show sucks (execpt the end title theme... that's pretty good).
I mean, take the villians for example. This is one of my biggest pet peeves regarding CP; they all pollute and destroy the environment because the enjoy doing so. That's apparently how Ted Turner views the world (or, more specifically, the US of A), and that whole point of view honestly couldn't be more idiotic. I mean, really; who in the world actually pollutes just for the fun of it? People litter because they're lazy, and corperations/factories pollute because they're cheap. Nobody actually does it for fun.
It goes without saying that Captain Planet is one of the worst cartoons ever conceived. They could have made it somewhat enjoyable (better characters, better writing, better plots... and that's just the tip of the iceberg), but they didn't even try. Had it been more of an action-based cartoon, with a friendly message at the end to remind people not to pollute, it could have been OK... but instead, we got... well, you already know.