I'm actually surprised it isn't going up over time. That is naively what you would expect as the low-hanging fruit is plucked. So the fact that it's been stable is probably a sign that scientific advances are roughly keeping pace with the (presumably) increasing challenge of finding ever more targets for drugs.

I actually do this for a living within pharma now! There's a TON of work that goes into drug discovery before we even call it a program. The odds of success are low, so we put in months of work evaluating a candidate before even have a hunch of a program.

Yes, the science advances, previously high-hanging fruits become low-hanging become high-hanging again [1], but the tooling also advances: we now have databases like OpenTargets which let us more easily evaluate potential drug targets. Failure is so much more than that, though: a program can fail after you've shown efficacy in animals, sometimes it just doesn't happen in the human subjects. Or you fail to find the right measurement (endpoint). A million ways to die.

[1] Gene editing is an example: impossible, then very possible, but now the blocker is public perception which in turn blocks investment.

I suspect there's a ceiling on failure rates people are willing to operate under. A diversified investor might still be willing to take high risk/reward bets. But workers can go their whole career producing nothing of value. It becomes hard to tell the difference between smart, hard working people who were unlucky and lazy grifters. I've done 2 series-A startups and I don't think I can do another one.

He points out that it basically has gone up, since the 80s and 90s brought some new therapeutic targets and approaches that by now have been mined out.