It's a matter of statistics. There are many people of comparable ability to the PCs who could have figured something out, such that it is improbable for them to have not done so already.
That the players only think for their PCs, while the DM is left thinking for everyone else in the world, would otherwise skew the rate of discovery to favor this small group over everyone else in the world combined. You need to give your NPCs some benefit of the doubt, because some of them should have figured this stuff out by now.
I agree that it's statistical improbable and requires some suspension of disbelief, but to me assuming that something was common knowledge but it was not used by anyone (PCs or NPCs) until one of the player figured it out, could be even more improbable depending on the context. It's in the end a matter of personal preferences and gaming styles.
Also, I feel the "There are many people of comparable ability to the PCs" argument is a double edged sword: use it too much and the players will start wondering why does it always fall to them to sort things out? Depending on the tone of the campaign, I don't see any issue with players coming up with "signature moves" or their characters.
Because unless you are int 20 and wis20 or have many years of fighting or magic there are quite a few npcs smarter, more experienced abd more savvy than you.
Did no god of war or arcana figure this out... Ever... And pass it to their prized followers?
Did the folks who invented those spells the player just learned not figure it out... Ever?
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Raw attitude and experience are not the only thing that matters in getting results. There are a lot of things that influence the final results. A smart person could solve a particular problem in a couple of months while a less gifted person could require 1 year to do so, but if the former has more pressing/interesting issues to work on there could be an opening for the latter. I've personally seen this happen quite a few times in my line of work, so I don't find it so outrageous.
Gods could be locked in a cold war state: direct intervention and passing of knowledge could lead to retaliation or disruption of the balance, etc.
There are several real life cases of people (even those widely accepted to be geniuses) inventing something and failing to understand the implications of their discovery.
Bottom line, I feel that you can find in-story reasons to both allow and prevent the PCs to be the inventors of something, and in the end the choice of one or the other is matter of playstyle, group preferences, settings.