Not really true, its just that the tights/knee pads/other lower body wear tends to cover up alot of the leg muscles.
Take Cena for example, he hits squats and leg presses hard every workout, but the jean shorts he wears in the ring makes it hard to notice just how big his quads really are.
Im not saying all wrestlers train their legs enough, not even all bodybuilders do. But I can tell you that thise who are really into it (Cena, Batista, Triple H) all have pretty big legs.
I'd say most WWE superstars train like bodybuilders for some time.. then they reach a size that they don't need to pass.. no Ronnie Colemans doing backflips. Cena has monster legs.. Mark Henry had tree trunks.. Let's remember Bill Kazmaier wrestled WCW back in the day.. top couple strongest legs of all time there.