On top of all comments generally being copyrighted by their author automatically, the licence at the bottom of a comment is like a no trespassing sign. The sign itself doesn’t stop people from trespassing. You still need to call police when someone trespasses. If you never call police then the sign is literally useless.
The licence is the same thing. If someone includes it at the bottom of all their comments, but never launches legal action when someone violates that licence agreement, then it’s literally useless. Given that launching legal action is incredibly expensive, I highly doubt the people using these licences will ever follow up. Also, how will they even know? How will they know a company used their comment as training data for their commercial AI? How are they going to even enforce the terms of the licence?