Why do predatory journals invite people to editorial boards? I've been invited to the editorial board of a predatory journal. I know not to accept the invitation or reply in any way shape or form, but I don't understand why they are doing this, which is a bit concerning. How does the scam work?

I assume board members don't pay money to the journal.

If they want me to make them look legitimate, can't they just copy my photo and biography from my employer's website and falsely claim I'm on their board? Why bother emailing me and bringing them to my attention?
Well, if you agreed they would not need to fear that you demand them to remove your photo and name and that you could sue them. Also, you could write "on the editorial board of…" in your CV and at some places in the world this would enable you to reduce teaching or get a raise. I think that's basically it. If this would be true, there would be some people that accept such an invitation. Not sure if this is the case, but does not seem impossible.