Sometimes a few examples are useful. Here’s 3 below.

It could be that and it could be that caused by a seemingly inconsequential change in something else. It could also be that overshooting by the wrong amount at the wrong time does it. That the amount some people need is consistent over time and for others it could be highly flexible. I could keep going with examples. The human brain is that complicated. Everything is best educated guesses and we turn them into prescriptions when those are right enough of the time without causing negatives that we can’t have.

Take the most complicated mechanical/computer system in the world. It’s complexity is at a minimum orders of magnitude less then the brain.

Our fundamental understanding of cells is so low that we spend a lot of money every year paying research groups just to try make some progress on understanding why a cell moves the way it does on blank surface. That’s a single cell, when you had in even a second thing things get drastically more complicated. I can’t give you exact yearly dollar amount but my grad research was on that and there’s quite a few journal publications based solely around that. Medicine moves forward because even without understanding those fundamentals we can still get a lot of useful stuff done.