I am almost always the only one who does so.

Makes no goddamn sense, you get a ticket for driving without a seatbelt on in a car yet it's perfectly fine on a bus?

Without a dual wheel axle all of them should be banned. of course the church that that their faith would protect them. Good luck.

being tax exempt

A lot of people put passenger car tires on their Van which are not designed to handle the weight of a fully loaded 15 passenger vehicle, because their much cheaper than truck tires. I drive a Dodge 2500 extended van and the only tires that are ever put on this or any other van I have had are 10 ply top quality Truck tires.
Way more expensive, but my life and the life of others is well worth it.

that our school bus didn’t have seatbelts. I would definitely wear one in a van.

Maintenance is often iffy with multiple drivers and no one makes sure everything is safe. And they are often driven by inexperienced people who don't enforce safety rules. Driver should be assisted by another adult who makes sure everyone is belted in before proceeding.

and hotel shuttles

These type of vans crash regularly it seems like

I NEVER ride in one without fastening my seat belt, even if it only going a short distance.

Anyone who fails to fasten their seat belt in ANY vehicle that has them is a potential Darwin Award Winner.

Get ready to duck!

I can tell you from installing carpet for 45 years what it feels like to have about 3,000 extra lbs in the back of a van. Luckily that load sets on the floor of the van so that helps. But...if loaded enough the front wheels get light to the touch because of all that weight back there. Most of the time it was just slow city driving but if out on the highway I knew to take it slow and start slowing for an exit ramp way,way before you got there.

to lobby for laws that they can insist on compliance to. That they apparently haven't yet suggests such accidents aren't common enough to make that desirable. Insurance companies are a big factor in both passage of and compliance with safety regulation.

30 years ago (!) I worked in property and casualty insurance, at one point for a brokerage specializing in church and private school clients. Who was allowed to drive was huge, and a few other things, but not wearing seat belts.

Clients were required to conform to risk management requirements, though. If we developed doubts and couldn't inspect, the company could and would fly someone out and either arrange for immediate compliance under supervision or cancel coverage on the spot. Ex-clients would then have to explain to new prospective insurers why coverage was cancelled, so compliance was the usual result.