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"?)