Is the notion that it is proper to do so, because everyone else has been doing it for a long time, insufficient? If yes, why is it insufficient?

I tip the van driver because they're working a job in the service industry with an established societal norm of tipping. That's why. There is no need for any further justification.

But I do have some ancillary reasons for doing so that not may not be a factor for everyone who tips. One is that I get enjoyment out of giving. I want to feel like I brightened someone's day. Another is that I want my company to have a positive relationship with the van drivers that carry our crews many times each day. And, finally, I've talked with a captain who himself worked as a van driver. He always tipped more than the standard $1/ride. His perspective was strong in forming my feelings on the subject.