I'm not even sure this is true. I suspect this particular expectation is strongly genre-dependent, and also dependent on the story being told.It's just a harder slog for the writer if they have to go against reader expectations.
I mostly read mystery for 30 years. Yeah, there were lots of nice romantic thrillers. But there were also plenty of bestsellers that didn't hook up MCs. Sometimes I might think two characters would do well together, but it didn't make me hate the book if the author didn't go that way. (That's what fanfic is for. )