Maybe it's because there's no reason to restrict them to the usual dimensions of real life. If there's a demand for big toys (and clearly there is) why not make them?
Personally, I like variety. While I wouldn't really want to date a guy who had a penis as big as my biggest dildo, it sure is fun to play with something that size every once and a while.
Not all toys are unreasonably large, either. One of my all-time favorite dildos is the O2 Mark, and it happens to be the same size of my boyfriend's penis.
Lots of toys are made with smaller-than-average dimensions as well. It's a whole spectrum, you can't just focus on the huge toys and think that's all there is out there. Some people like big ones, some people don't, and there's no reason not to indulge the desire for a pornstar-sized wang now and again if one falls into the former category.