Like bars?. That was the first link that came up, and folk are saying they look good, but very expensive... Considering they're about half the price of some of the MTB bars out there I'd say that it's at least in part because MX guys aren't as interested in chasing lightweight through things like expensive bars. I imagine more gains can be had by spending money on the thing between your legs that makes it go.
Do they make anything that could save considerable weight though, like the chassis? The answer is, a big resounding no.
Dunno, might be something to do with the bikes weighing 10 times as much and travelling several times faster, and having many times more power so less concern for shaving grams? Even then, some moto-x parts are carbon.
Motorcrosses and Superbike riders are all happy to save a 1kg of weight. IMO carbon in mountain bikes was simply brought over from road bikes to appeal to the types who play golf and drive Audi's. In fact....mountain biking and road cycling is the new golf.
I'll go carbon when the two wheel motorsport world starts heavily using carbon for stuff like the chassis - instead of one off's that seem to continually fail (Ducati).