I don't, running YouTube is expensive, and Google became a trillion dollar company mainly from ad revenue, it has to be making bank for them to not have killed YouTube
YouTube was intentionally run at a loss for years in order to quell potential competition and set up a monopoly. Now, they got all the content on their platform and that won't change for a while, so they can do whatever they want. It was a marketing strategy, not magnanimity.
A similar strategy was led by HP. They make no money from printers, but once you buy one, you are very likely to also buy their ink which is sold with a ridiculous profit margin, so that the loss of revenue from printers doesn't matter in the slightest. It's called "loss leading".
I'm not talking about them taking loss in earlier years (that aside, even if they tried to run it for free in the current year the number of users is vastly different)
Video streaming is expensive in general, especially when it's not a fixed set of videos (like other streaming services)
You'd be surprised. Half the cost of premium goes to creators, and making $5-7 per user per month through ads is definitely doable. That equals pretty much exactly the cost of premium