Reviewers work for free for a few reasons reasons. First, it's prestigious to be a reviewer for a top journal and very beneficial for getting tenure. Second, it allows you to create connections with other top researchers. Third, it's giving back to the community by doing a service that needs to be done.
My thoughts exactly. I've never known a journal to do anything to promote the fact that someone has reviewed for them (eg produce a letter on request that states which articles someone has reviewed for them). It'd be a nice and free way of creating value to reviewers who generally work for free, but I don't know any journal who's ever done this.
