I'm known to wear loose shorts while mtb'ing. I like tights or lycra shorts better because they don't catch on things like the horn of the seat. I hate it when I mount my bike and the bottom of the shorts catch on the horn as I'm fixin' to take off. The same when dismounting, I hate when the shorts catch on the seat and it looks like I gored myself and kinda giving myself a wedgie. Tights and shorts are a good no-nonsense piece of clothing for bike riding. They wick moisture away from the body too.
Lastly, tights and lycra shorts stay in place. When I bicycle at top speed, loose shorts will billow up the thigh, giving the ladies too much leg to look at. In my case this can start a stampede and we can't have that on a busy street can we?
Ernest