In that case, why don't they allow competitors to host their videos?

I took the parent's comment to mean they would rather 'free' viewers leech off other people's bandwidth, as if they're not Premium and not watching ads, they are just consuming bandwidth and costing resources.

If they go to a competitor and block ads and consume bandwidth, then they are actively costing the competitor rather than costing Youtube for hosting.

Youtube allowing competitors to host would just consume youtube's bandwidth but with the 'views' also going to the competitor's domain (so the competitor could then potentially get additional sponsor support for having 'eyeballs' on their domain).

My point is that they intentionally obtained an effective monopoly on content, so it's weird of them to complain about too many people wanting to watch content there.

How exactly did they obtain a monopoly on content? They don't name most content creators sign exclusivity agreements.

I said an effective monopoly. Most people who create a video will automatically upload it to YouTube without even thinking about alternatives.