Or they need less-agile guys who are able to take a hit.
Heck, in my last game, I got hit/run into by players on 4 separate occasions. Luckily for me, I'm 6'4", 270 and I can absorb it. In one case, I think I (accidentally) knocked the guy over. I even had a couple of players apologize to me for running into me.
In all seriousness though, agility and mobility are a big asset to the umpire, along with being able to anticipate what's going to happen. But there are times where you'll still get caught in a bad spot, and there's not much you can do about it.
I've heard rumblings of possibly moving the U to a position behind the offense, opposite the R. I'm not sure if I like that idea or not. It would be a safer position, to be sure, but I'm concerned that we would be less able to see the action that we need to see to officiate properly.