Why aren’t corporations analogous to unions? They’re an organization of people who work together as one. When was the last time you heard about a bidding war between two different managers in the same company trying to hire the same person? It doesn’t happen because the company organization doesn’t allow it to happen. This is exactly the flip side of two union members not undercutting each other on pay because the union organization stops it from happening.

You say that corporations aren’t created for collective negotiation. When was the last time you bought something from a corporation? Did you negotiate with individual members of that corporation on the price? Do you get individual cashiers to bid for your business? Of course not, because the organization they belong to insists on handling price negotiation in a consistent fashion across the company.

I suspect a lot of the negative attitude toward unions comes from an incorrect assumption that a union must be a monopoly (or perhaps monopsony is the right term). I think that’s why you immediately jumped to “cartel” for the analogous organization on the other side. There’s no reason a single union has to represent all workers in a field, any more than a single company has to be the only employer in a field.