Why ? I've worked on compilers and theoretical programming languages and as such, I have notions of a dozen programming languages. I don't claim to be good/expert on more than 3 though.

But if someone does this on occassion as a hobby, how is it weird to know, or have some projects in 10 or even 20 programming languages ?

Also many projects are programming languages in themselves (Greenspun's 10th law). Tensorflow, most scheduling packages I've seen, prolog, several things I've written ... Once a project grows beyond a certain large size, it tends to become a programming language in itself.

Perhaps I was too general. A more accurate statement would be if more than 3-4 languages are listed without any distinction between languages you're good at and languages you've just used, then I become skeptical. And I said skeptical, not that I reject the resume outright. I just find that more often than not those candidates don't actually know more than one or two of those languages very well. Or worse, they don't know any of those language very well. (Not that this is a bad thing! We all have to start somewhere; but listing a lot of languages in such a situation seems dishonest from my point of view).

This is false negatives vs false positives territory. Outliers can get caught in the crossfire - nothing new.