What's up with the school buses?

I had a meeting at the middle school this morning, and I was leaving around the time that most of the kids were arriving.

Actually, the building was packed with kids, the ones who arrived by bus.  Out front, though, was a long line of cars dropping kids off in ones and twos.  Each child would dutifully wait until the parent's car pulled to the front of the line before getting out.  I saw no child get out of a car more than twenty feet from the front door, and there was a line of cars spilling out onto Manheim.

I assume that this is a daily occurrence.  I don't get to the middle school all that often, so it was new to me.  It makes me wonder, though, what the problem is with the buses that so many parents drive their kids to school.  I have some theories:

• The walk from the bus area is too far.  This is possible, since no child was traveling as much as fifty feet from car to door.
• Bullying happens on buses.  I know it did when I was a kid.
• The time on the bus is too long from some parents (or children).

Beyond that, I have no clue.  Are there any driving parents who can tell me why they do it?  It seems like it adds a lot of time to an adult's morning routine, adds even more pollution from idling engines, and doesn't utilize buses that the district has to run anyway.  Are one or more of my guesses correct?  Is there another reason?