I am not sure the problem is that YouTube doesn’t care, but more they are constantly fighting to keep up with the spammers. With some of the automation tools around dropping a heap of comments is rarely difficult, either as a single big push or drip feed overtime. These accounts by the sound of it are essentially burner accounts, they are easy to create and have little value if they get banned or removed.

On the other hand the accounts that are posting the videos are worth more and likely they are willing to pay for an aged account that is possibly already enabled for monetisation. It would depend on the amount they pay vs the amount they can make in the period before the account gets shut down. I suspect even if each account only lives a short period they are still probably making a nice little profit from each one.