Yes, dropped floors might be expensive, but my condition is progressive and I need something that will work for me in the future. I have Muscular Dystrophy, so my muscles weaken over time. While I may be able to transfer now, I probably won't be able to in 2-3 years.
In my current setup, I load my chair on the back of my Jeep and then walk around to the drivers seat and get in, but I'm losing that ability and that's why I need a van.
For other chair users, everyone is different but it's easier for people with higher level disabilities to stay in their chairs. They often have specialized seating systems and a regular car seat wouldn't support them properly. Plus it's nice to have your chair with you when in the wilderness.
For window visability, you can't really get bigger windows. It may not be such an issue for manual chair users, but for big power chairs it is. Most wheelchair vans have the lowered floor.