Changing jobs always carries risk - the new company might not be a good fit, and you may not last there very long perhaps for reasons beyond your control. Likewise, there are many jobs - e.g. early startups - where the risk of the company itself going under may be significant.

People in the country on an H1B lose their lawful status immediately after their employment ends, which means being unemployed for any period of time carries risk of deportation. Most people don't want a change in their work situation to translate into being forced to leave their country of residence.

Between these two points, I'd expect people here on an H1B to have a narrower field of jobs that seem attractive - high-risk startups aren't going to be appealing to most, and to generally be less mobile than those who don't fear deportation if a new position doesn't work out. Because of this decreased mobility, employers can get away with paying less.

Because some of us live in the real world and see this literally happening every single day?

Every single day? May be you should come out and meet more people, just saying. Again, you might come across certain incidents where this might be true and there are cases, where they are more educated than an average American and get paid more. But to generalize very broadly is something won't help people properly understand the issue.

Someone on H1B usually is also waiting for employment-based green card or promise to file for one. Going to another employer means that this multi-year process needs to be restarted.