While I can agree with the 4K argument a bit for the hosting is expensive part, I do think it goes way beyond that or long videos. Youtube exists on a scale most don’t even realize. Tens of millions of people upload content every day. We’re talking thousands of hours worth of video every day.
Not to excuse YouTube for a lot of dumb decisions they made, but even on their best day the site just breaks even due to the cost of hosting.
It’s the two that aren’t outside in the snow.