Refusing to provide breaks is illegal, true, but it's trivially easy to legally force employees to take many fewer than they're entitled. Amazon employees in FCs don't piss in bottles because they're very into logistic efficiency of picking...
True, there will always be entities that abuse understandings, employer and employee. The thing is, corporations are superentities with monopsonic power. Employees are units, discouraged from communicating, with limited time in a day to make complaints or study potential improvements to working condition (or law, for that matter).
Unions are a method for setting up a superentity that has the bargaining, informational, and legal power to have a more equal footing with the corporate superentity.
Particularly when Unions have substantial ownership of the company, they have no specific wish to lose dues-paying members or make the company fail.