One could make a DH stem adequately strong with less material by sticking with shorter lengths, but I think it's more about control. Other things being equal, a shorter stem makes it easier to get weight off the front wheel. It also reduces a bike's tendency to nosedive under braking, or on an abrupt transition from a jump or steep section. Sometimes it's desirable to have more weight on the front wheel, but that's really not difficult on the way down.
Cross country riders spend more time climbing than descending, on most trail networks. A good XC setup facilitates climbing too, and it can feel awkward to try to weight the front wheel or climb out of the saddle with too short a reach. Too short a reach also murders the rider's back. Too long a reach does too, and stem length and height is used to fine-tune this. Honestly, I'm not sure if there'd be a negative consequence to a long frame with a short stem. My instinct is that it would be twitchy. Certainly that's what happens with too short a stem on a road bike. DH bikes have very slack head tubes and most DH riders use very wide bars, so a short stem wouldn't make such a bike twitchy to the degree that it would screw up the handling on something with a steeper head tube and narrower bars.
So, I guess what it comes down to is that for neutral handling on a bike with a fairly steep head angle and relatively narrow handlebars, a longer stem is necessary. For getting weight off the front wheel, a short stem is helpful. Reach is important too, but it's also effected by bike size.
A trend recently is for XC and AM riders to use short stems and wide handlebars too. Fisher bikes historically have run a little long, and used a shorter stem and slightly differently-dimensioned fork. So you can see there's a few ways of thinking.
Are you trying to choose a stem for your bike?
Too much for me - all my weight got dumped on my hands and my back started to hurt. Lame. I liked the uphill handling, though.
