Researchers dont get paid. In fact, they usually have to pay to be published on reputable platforms. Those platforms are the ones charging.

As for why they publish there if they never profit, academic success is dependant on publishing (“published or perish”), so academics will pay to advance in their field, potentially getting funding for future studies. (Not from the publisher, obviously; but from grant programmes that only fund you if you have a body of work already.)

This is why if you contact a researcher directly, they’ll likely be willing to give you the finished thesis for free.