It's part of the culture. I write a paper. I want to get it published. Journals need someone to review it. If I refuse to peer review others' papers, then in principle people may refuse to review mine and we both lose.
Sure, it'd be nice if publishers passed some of the profits to both authors and reviewers, but that would create other perverse incentives.
> and what real care do they take when they are doing it for free?
Not much. They sometimes sit on it for months before reviewing it.
