Because of regulatory rules, there isn’t much they can do about the extent of preboarding (which I agree is sometimes outrageous). The rule is that the airline has to offer a preboarding opportunity, and they are not allowed to inquire as to the basis for the preboard. I once saw an entire group of about two dozen or so able-bodied police officers on the way to a union conference preboard as a group - that was a substantial percentage of the capacity of the CRJ we were on.