Again: a sense of duty, it's considered part of their job. They can certainly choose not to do it, or not to do it for paywalled journals. However, when I suggest that, I often get the reply that that's not fair to those who submit to those journals - often early-career researchers whose career could be greatly helped by publishing there.

And again, I can't fault them for that.

(I assume you meant to say "don't benefit"?)

I don't buy the sense of duty argument. It's good for the career.

Until the appearance of the notion of open peer-review, the reviews were not public and thrown away by the editor after the publishing decision.

Concerning the sense of duty, nobody stops anybody to review a work and post it somewhere publicly accessible. However, that's rare to see.