Youtube benefits from being everyone's go-to site for finding video content. Keeping that monopoly of mindshare is worth far more to them than any short sighted profit maximisation.
Yeah that's not some kind of general rule. This is because the US legal system in particular lets this happen. And in fact, the US legal system works so badly that it often still benefits companies long after they've monopolized too much, causing all sorts of damage to consumers and other companies that actually did nothing wrong.