If I sell a novel to Simon & Shuster, and assign the copyright to them as part of the deal, I can't sell the same novel to HarperCollins under the same terms. I can't sell my movie to Disney AND Sony.

This is the same, but the price is zero. You can take or leave the deal at that price, and you'll probably take it for sufficient academic prestige points ('impact factor', visibility, etc).

By the way, some publishers such as IEEE explicitly allow you to post preprints for free download, eg. at arXiv and on your own web site.

Nobody gets royalties for published papers.