That's because, as you've stated in the rest of your post, you aren't doing what I was commenting about.
You don't specialize in service work, which was what I was talking about. You don't stock all colors that you may come across. You don't carry many of your tools with you that you may find yourself needing. Etc.
I've found customer's descriptions of their problems to be horrible, sometimes you arrive to find a completely different problem than was explained to you. Also, many times a customer thinks of something else that they want to get done. If I go out to a service call 20 minutes away from the shop, and I end up selling something extra (maybe something that requires a bit of pipe outside), I want to be able to drill that 1 3/8" hole through the foundation and bend the PVC to fit. I'd like to bang out the job at that time, instead of going back again which wastes time and fuel.
Having to go and get something just once per week could easily cost more in labor (lost time that could be spent working) and fuel than the fuel savings of a Transit Connect over a typical E-250 or similar.
Actually, I did service work for years, I still do it when called to and I'm very good at it, but there isn't service work in my area like there used to be. Home Depot saw to that. I can't even remember the last time someone called to replace a light socket or a switch.
Colors are a very local thing. There is one region I've done work in where you find a lot of Almond devices, but nowhere else. Brown is almost unheard of anymore, as is Ivory. Times have changed.
Actually, the only tools I don't carry are shovels and my TE-60, but I wouldn't carry them on a full size van either. I have had to loan guys in full size, well stocked vans tools that they didn't have on numerous occasions. I've never had to borrow a tool myself.
Customers' descriptions can be horrible, but it's better to have them than not to have them. If I have a service call 20 minutes away, I'm in heaven. Most of my work is 40-60 miles away in an area that's poor in suppliers. I recently needed a decora triple single pole rocker in light almond for a fan/light/night light combo and had to travel to two supply houses and a HD to find it. Total: 20 miles driving. Will I stock one on my truck? No. It was the first and only time I've needed a switch like that in six years.
To your last point, even with an E250 you will run up against something that you have to go get from time to time if you do resi service, and many things you'll need to order in anyway so you'll have to make return trips (fan motors/blades/squirrel cages, fan controls, lumiline and other lamps, smart switches, electric heater contactors, TVSS receptacles, undercab lamps, various pullchain switches, low-voltage sockets, etc. all come to mind). If you're only 20 minutes away from a supplier or your shop, it's really not that bad. You may not feel the fuel pinch because you work so locally. I don't, and a full size truck would eat a few hundred dollars per week. A buddy of mine who does phone and network jobs out of a F150 was spending $1600 per month just on fuel. He got a Toyota Prius for service calls, which make up a large part of his business. Out of that $1600/month he was paying, he has about $1000 savings plus a free vehicle. He still uses the F150, but only when it's necessary. That's over $10,000 savings per year, not an amount to sneer at.