It is probably becausd think companies and especially corporations have proven themselves too effective essentially for that in the pragmatic sense. Regardless of the many objections and insults to them in the generic, and claims made about the utopian alternatives and costs as better corporations and their byproducts are implicitly accepted as the most practical option even by their foes. As a category they are /never/ treated as an underdog in a comparison to any other entity and avoiding involvement in either goods or working for them is essentially treated as a luxury.

Of course the comparison doesn't quite work because unions are not standalone entities - they are linked to government or companies. If they were independent the labor organizations would be guilds.