Bandwidth costs money, YouTube can probably do it cheaper than end users at scale. But this isn’t about reduced bandwidth expenses, this is about maximising profit extraction.
No, the cost of bandwidth is less than their subscription value, presumably providing this option would result in less profit despite lower server costs, but they already have a global CDN so I assume it’s cost is relatively low anyway. I imagine if this was a path they wanted to go down, they could crowdsource it via chrome directly, without providing any positive value for users.