My guess would be that if EVERYONE drove their kids it would be an impossible situation to enter the lot, drop off safely, an dget the kids to their classroom. Much as with adults, carpooling is better for a lot of reasons. Think of how crowded open house nights are!!!
Buses also get the kids to school on time. Can you imagine how many parents get their kids to school late because they live on their own schedule? At least if you commit to meeting the bus, you are more likely to do it then when you are just worried about your own timeline.
I aslo think riding the school bus is safer, but that is total opinion and I don't have stats to back it up. The buses are high up, large, and slow. School bus accidents are rare. Mom, in a 5 year old car low to the ground, distracted on a cell phone, with a coffee, yelling at her kids, is far more dangerous. I'm sure you are a great driver :) but many parents aren't and if I were a principle, I would want as many adults and their cars away from my school as possible.
By all means, keep doing what works for you, but I suspect those are his reasons. I like the bus, because I hate driving in the school parking lot where there are people walking and parents cutting in the carpool line. I like to tell my kids we are being environmentally friendly for not driving our SUV on such a short trip. And we enjoy the walk together and hanging out in the morning without the stress loading into the car and driving. Although, I don't like those zero degree mornings in January..