"not that expensive" is relative; it's still a lot of money. Sure, it's not trillions of dollars, but it's still billions of dollars. YouTube has historically not returned a net profit (and I haven't heard of that situation changing).
YT financials and P&L were not broken out in audited financial statements back in the day.
Allocating costs for things from Google that they use (e.g. the Ads system) is difficult. The problem with any subsidiary.
I have premium, my wife accepts all of those ads
I'm sure that's what Google's accountants would love us (and the IRS) to continue to believe.
and also that streaming and storing video at that scale is almost a natural monopoly, with how much it must cost and how hard it would be to compete without existing resources