Because it would end up favoring research that may or may not be better than the honestly submitted alternative which doesn't make the cut, thereby lowering the quality of the published papers for everyone.
If they're using AI for reviews that's already the case.
That can't happen unless reviewers dishonestly base their reviews on AI slop. If they are using AI slop, then it ends up favoring random papers regardless of quality. This is true whether or not authors decide to add countermeasures against slop.
Only reviewers can ensure that higher quality papers get accepted and no one else.
It ends up favoring research that may or may not be better than the honestly reviewed alternative, thereby lowering the quality of published papers in journal where reviewers tend to rely on AI.