It's the opportunity cost. When you provide an employer with unpaid labor they profit at the expense of your family/hobby/startup/charity/mental health.

Your extra time could be devoted to the benefit of people, projects, or causes dearer than your employer's pockets.

If I work them them, I'd rather the company fire the one who wants to routinely work 80 hours a week. It's not helpful -- and I don't want to waste my time dealing with their code.

If people want to work long hours then fine, I've no objections to them doing that. But in my experience most people - outside of a very small number of vocational jobs such as farming or being a doctor - don't want to work long hours and have other interests in their lives that they wish to attend to.