A lot of the things that unions do when they "bargain collectively" are things that, in fact, large corporations aren't allowed to do. For example, SAG-AFTRA us

A lot of the things that unions do when they "bargain collectively" are things that, in fact, large corporations aren't allowed to do. For example, SAG-AFTRA use the fact that big-name actors are members as leverage in order to force productions to employ only SAG-AFTRA members, who in turn have to agree not to work for non-union productions in order to get work. This is a lot like something that for example Intel got done for in multiple countries - using their control over the supply of laptop processors to strong-arm companies into buying their desktop and workstation chips from Intel too - except with even more restraint on trade, but it's legal because they're a union. Other typical union goals include ensuring that every single hotel, conference center, etc in major cities make use only of their members' services at their fixed prices and ban exhibitors from doing any of those things themselves. If a company or group of companies did this, they'd be in huge legal hot water, but for unions it's fine.