It's not about 5 year return, it's about the fact that the potential cancer cure simply has low returns and huge risks. People are absolutely willing to make long term bets (e.g. snapchat, which has negligible revenue right now, and is entirely just hope for the future), they just need to be good bets.

Molecular targets are getting harder to find. Getting a drug through the FDA is insanely difficult, costs huge amounts of money, and often fails. Further, there is significant political risk once you get it through - very few countries will pay full price for your drug (mostly just the US), and who knows where the US is going?

Pharma is both a scientific and a regulatory challenge - making money involves the stars aligning.

If you want more cancer cures, make it easier to become very rich by selling them.