That's not necessarily true. In order to remain viable, a company needs to make a profit, or at least break even. If Costco is already at the point where they're giving as much to their workers as they can while remaining viable as a corporation, then a national retail workers' union isn't going to help Costco employees directly.

This is a 'feature' of capitalism, and has a lot to do with the way that national and global economic events play out. An individual corporation may or may not be able to do much about the market conditions that it exists in.