We (public) don't know that. Google never splits YouTube's costs out, only revenue, and the only reason I can think of is that it's losing money. IMO it's highly likely that YouTube costs a lot to run (just imagine the costs of storing zettabytes of videos, 99.9% of which will get watched less than 10 times ever, in 4K, and be ready to quickly stream them anywhere in the world) and Google lose money on it, but compensate with user data they use for their wider ad business. People complain about YouTube ads today, and how expensive Premium is; maybe a future independent YouTube can cut costs, shed some old/unwatched content, and become profitable.