BJJ classes

Typically BJJ classes start randori or "rolling" sparring on the ground. why dont they start standing up like they do in most tournaments?

Cuz the ultimate goal is to do 'your' job on the ground. I agree with you matt, they should start standing.
there might be more of an empahsis on takedowns then, which would make BJJ guys that much more dangerous. That's why wrestlers are so good at mma, if they can only get past the feeling of being on their backs. Look at Matt Hughes, once he realized being on his back wasn't so bad, he became a champion. On the other hand, look at Brock Lesnar, he still does everything he can to stay off his back like he's gonna be counted out or something...

From my experience, it's from a safety point of view.

Usually there is not enough mat space. One covers less ground when one starts from the knees, and there is less of a chance of getting a foot or body landing on your head. We still get that once in a while even while starting from the ground.

In our competition class, which is smaller and for higher rankings, we do start from standing and often the first round is simply a throwing round. Even in that case, the 200+ lb white belts are moved to the warm up mat space for safety reason. As a featherweight, I appreciate that.

safe training is first! I got some HUGE bumps on my had just from someone kicked me on the head whil he was sweept. I don't want to get a heel from someone falling from a judo throw...