https://stacks.math.columbia.edu/
The people who are looking up references to advanced math concepts are likely students who are already on a mainstream pedagogical pathway and are looking to fill in holes to a concept map they're already building.
The use case of someone who (1) does not wish to consult the vast and well-discussed pedagogy of math and (2) is not an advanced math student and thus wishes to have stand-alone math definitions is a Very special case.
Okay, but I don't think those communities are relevant to this conversation.
> The use case of someone who (1) does not wish to consult the vast and well-discussed pedagogy of math and (2) is not an advanced math student and thus wishes to have stand-alone math definitions is a Very special case.
Number 1 is a weird assumption! Unless by "consult" you mean spend weeks studying a textbook, the problem is that consulting is too difficult! And if I understand "harsher" correctly you just said the other sites are harder to use, didn't you?
So then it's just "not an advanced math student", which may or may not be a majority of people on these pages but it's a very significant amount and it's the more important target for a general encyclopedia.