Even the most "solo" proofs rest on a foundation built by others (notations, concepts, lemmas, entire frameworks invented decades or centuries earlier), so nothing really is created in isolation.

It's also right to point out that mathematics can be done in solitude — but in my experience, that solitude is anything but passive. It takes a kind of disciplined internal dialogue—working through examples, forming your own structures, asking endless questions. (I quite like the way Paul Halmos puts this: “Don’t just read it; fight it.”)