You cannot prevent cheating with other policies like the Developer Certificate of Origin either. Yet no one brought up the potential cheating at the time these policies were discussed.

Several projects have rejected "AI" policies using your argument even though those projects themselves have contributor agreements or similar.

This inconsistency makes it likely that the cheating argument, when only used for "AI" contributions, is a pretext and these projects are forced to use or promote "AI" for a number of reasons.