There is a difference between doing fundamental research and bringing a product to market.

We know enough about quantum computers to be able to build some of them in limited ways which are commercially useful in very specific domains, so then companies can get investment to do that.

But we still don't really understand all of the fundamentals, and it's always hard to raise investment for fundamental research.

Suppose you could prove that building a quantum computer that can run Shor's Algorithm on interestingly large numbers is impossible. Knowing that would be of immense value. People could stop wasting time trying to create it or defend against it. But how would anybody commercialize that information? Who has the incentive to fund that research?