This seems to presume that the members of this group have common interests to negotiate on (they dont) and to ignore group psychology. If I have high individua

This seems to presume that the members of this group have common interests to negotiate on (they dont) and to ignore group psychology. If I have high individual negotiating power and I want to, say, work from home then I do not benefit from unionizing with a majority of people with low-power who do not want to work from home. it's quite plausible that the minimum common interests the group has are already satisfied by the employment terms, and then, the negotiation is an opportunity for "union reps" to game the system to the advantage of some over others. You iterate this over a large number of negotiations, thrown in "union" as now a group and not a mere negotiating block, and you get the police union.