That seems counterintuitive to me. Why does there being other people employed by the same people prevent me and the group I'm in from entering into contract together?

Why are unions treated differently than other contracts?

I'm assuming this is because of some historical accident that came about with a struggle between people who wanted to prevent all such contracts and people who wanted unions to have special privileges over other sorts of contracts?

A majority union isn't a kind of contract. It represents all workers in a bargaining unit, even workers who voted against it. In theory, this simplifies negotiations and helps make sure all workers are fairly represented.

Some union supporters worry that allowing minority unions would weaken established unions. Union opponents have focused on passing "right to work" laws, which require majority unions to represent non-members for free. The status quo probably makes unions stronger in pro-union states and weaker in anti-union states.

Thank you for explaining! Your explanation is quite helpful.