Wait: so you're claiming that there's no money in selling games, but somehow the people selling games for no money can give lots of money to the people who wrote the engine?
It is about like selling pickaxes to miners during the gold rush. There are hits that bring in a lot of money. That money turns around and goes out the doors for fancy tools. Those tools are then setup somewhere where there isn't any gold and you end up a poor fool. As an example Ion Storm probably paid ID a few million for their id tech licenses, produced a couple of hits and a couple of failures. The failures were big enough to kill the company.
How does that work?