I think there's a false perception that unions negotiate against the companies best interest.

Unions negotiate for a bigger slice of the same pie against leadership, executives, and shareholders/owners.

They have the same incentives as those to see the pie grow, but band together to negotiate that their pie be bigger and those of the above smaller than what would have been otherwise.

Most of the time when it results in squeezing the company itself it's because leadership wasn't willing to share downsides.

And this is the primary reason for unions. When things go well, leadership is rarely willing to share upsides. When things go bad, leadership is often unwilling to share downsides. Workers join union to pressure leadership in sharing both upsides and downsides.