Why is nobody willing to take a risk to revive this market? If the only thing preventing a revival is cheap planes, couldn't someone get some investor to pay for all the certifications of their modern, fancy, efficient, cost effective plane, and then sell a million of them?

That is essentially how any new industry works. EV's were too bad/impractical/expensive until Tesla decided to take the risk and put down the capital to make them mainstream. Why can't/doesn't someone do the same for GA?