Alphabet don’t publish YouTube’s profit margins, so I don’t think you know that to be a fact. I’d personally be rather surprised if it wasn’t profitable though.
I know this is horrible logic here but: Alphabet not wanting to publish the margins of what is otherwise their top3 best known product says a lot in and of itself. Either that it wouldn't be a pretty image (even if it is in fact commodifying other profitable sectors), or it'd reveal some skeletons (which are being revealed in real time, but it slows down the reveal).