It's not just that people have worked on it for a long time, it's that there are dozens of problems that are known to be 3sum hard and people have tried to make those as efficient as possible too!
So every grad student who has thought about geometry problems will have spent more time than they're proud to admit thinking about better algorithms only to realize it's 3sum hard.
It's not just "people have thought about one problem a bunch" - the whole class has been wailed on to no avail
Eg here's a paper with a bunch https://people.csail.mit.edu/virgi/6.1420/papers/gajovermars...
So every grad student who has thought about geometry problems will have spent more time than they're proud to admit thinking about better algorithms only to realize it's 3sum hard.
It's not just "people have thought about one problem a bunch" - the whole class has been wailed on to no avail