It's probably not what you think - but a run that takes 2-3 minutes to complete, and you have eleventy-five different cars waiting to take a run...........
Not to mention the timing nightmare that happens when you are staging multiple cars to run consecutively, and the guy in front spins-out (ARRRRGH!!! I was all set to make FTD!!!).
Mostly, just the frustration factor - not necessarily some safety issue. At the SCCA Nationals last year in Topeka, most of the "fast guys" had runs in the 30-40 second range, and they staged cars ~15-20 seconds apart (mem'ry here).
Plus, if you have a couple of "crossroads" that divide the track, you can have 2 courses going at the same time - easily 4 cars running at any given moment.