Richard Feynman had a really good bit about how bad human intuition is about quantum physics. About how we evolved to throw a rock at an animal out on a grass plane, and not to make good guesses about the nature of particles so small we can't even fathom them.

Seems appropriate here.

The simple chemistry is pretty specific and doesn't work very well (it usually makes a semiconductor instead, and even when it does work, it's a few tiny impure specks most of the time).

Why is it unbelievable?

Why not? Lots of technologies are literally just about mixing chemicals together, and we could have done that at any point in time prior, we just didn't.

Lithium ion battery technology is literally just mixing materials together. We've had access to lithium for centuries but we didn't turn that technology into batteries until about 30 years ago.

Why not? The search space is massive, and it's not the first time humanity stumbled upon some simple revolutionary discovery.

Just because we failed to figure out the conceptually simple thing until now, doesn't mean it can't be the case. Anything else is just some form of human exceptionalism.