This article in the Guardian is definitely worth a read if you’re not intimately familiar with just how it got this way… It’s 8 years old so it won’t cover recent history but does give you an idea of how it started.

A very interesting read!

So, what i take form the article, is that Elsevier and other publishers are most similar to a search engine or index: They give you a list of all interesting articles in a field, so you don't have to search through the millions of scientific articles produced each year yourself.

That makes it kinda similar to google, which is also very profitable, which also turns a profit by giving back user-supplied content to the users. Just that Elsevier charges for that "indexlist" functionality directly, while google takes the game one step further and harvests data, which it then uses to display targeted ads.