Why don't they just charge [more] for a mountain license? A few thousand per hiker would probably be tolerated by people who view the hike as a lifetime achievement kind of thing.
There’s far more tourists in Nepal who are trekking than mountaineering. And those tourists are going to be much more price sensitive. It’s not just wealthy Americans, but people from all over the world - India, China, middle income countries etc. All those people are spending money in tea houses and hiring local guides.
They do charge quite a lot for mountain climbing permits but trekking is mostly just walking from village to village along the usual paths the locals use.