Because they keep working there. So the workers think the conditions are worth it in exchange for their paycheck. If I disagree and shop elsewhere I'm effectively disagreeing with the workers and telling them I think they are making a bad deal and I don't want to contribute to their livelihood. But I don't actually know that they can make a better deal elsewhere.
Before I move my purchase to Newegg or whatever should I first make sure that their working conditions are better? Or just judge by how often I see each company mentioned in the media?
I think that the voluntary nature of employment is a sufficient circuit breaker. If it gets bad enough we can and do leave for better opportunities elsewhere. It wouldn't be the first, second or third time for me, and that includes minimum wage jobs. So I don't feel the duty to investigate the working conditions of each business I deal with.