Why so few mountain bikers with spandex? I've never had clothing that was specialized for cycling, so I'm kinda n00b in that area.

I am confused about the use of spandex.

It's very common among roadies, for obvious reasons. It's more comfortable, has less friction on the seat, and is more aerodynamic.

Why doesn't it make sense for mountain bikers? You do see plenty of XC riders with spandex, but not in "120-160mm travel" areas like trail/enduro. Those guys seem to use basic shorts of one type or another, that catch air and might catch on branches.

Why isn't spandex just as common in the mtb circles? Is it a cultural thing?
As already said, aerodynamics are less important to MTB's, but otherwise its largely convention and fashion that dictate what people wear.

A vast majority of MTB'r are not wearing basic shorts - they are usually wearing shorts made for riding, including padding just like Lycra road shorts, flat seams and materials designed to withstand the rigour of riding. Many MTB shorts consist of removable lycra under shorts with the baggy outer designed for cycling.

For me personally, I wear baggies MTBing because we often spend time "in public" - around cafes or pubs after the ride, socialising and I feel like a fashion mistake (and I am not fashion concious) walking into a cafe in lycra. I will wear lycra if doing long distances, or I know I won't be more than 10 feet from the bike for the whole time I am out, but feel more comfortable wearing shorts.