I think your brother is probably rolling his eyes when he agrees that yes it would be great if we had a cheap and effective universal mass screening tool for cancer. I mean, why would -any- doctor not want that. Why would any cancer researcher not want that. Oh, but it would mean they would lose their funding you say. But in exchange, you get ever-lasting fame. Oh, and you don't have to worry about having probes shoved up your ass, or have bits of your breast cut out for inspection when you get to "that age".

I think your supposition ignores a lot of the latent difficulties in the subject area. You could very well rephrase that as "why not try to create a drug that just kills all cancer with minimal side effects as it seems crazy to have to cut someone open and cut it out by hand".

The answer to "why haven't we done more" is "because it's hard and we're working on it". Why isn't automatic machine translation any better? Why are natural language interfaces so crappy? Why does my speech to text program make mistakes? Why are we still using steel and concrete? Why don't have we have a room-temperature superconducting power grid? Why do we still have to get our meat from animals on a farm? Why don't we just make a cure for HIV? Why don't we just make a cure for the common cold? Why don't we just have a malaria vaccine (they really are working on this one! It's HARD!)?.

The answer to all of these questions is "because it's hard and we're working on it".