The question you quoted implies the speaker already knows that several/many people usually go there. The sentence combines "who are [they]" and "the ones who go" (= they), where both parts are clearly plural, in meaning and form.
(The way you tried to split it doesn't work.)
But when you ask the generic question "Who goes there?"
you don't normally know in advance if the answer will be zero, one, or several
(unless the question is rhetorical/sarcastic).
Hence the singular "by default". Does it make sense to you?
This is only a rationalization, not proper language history. But the mechanism is the same in Latin (and its descendants) and German, meaning it has been used for a long time.