Cooperative bargaining is a positive-sum game. If you and I work together to negotiate a contract, we can get better terms than either of us could get individually.
They don't have to hurt everyone -- there are definite good unions -- but there are bad ones too.
Example: Look at the police unions now.
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.