Why is being single not a happy ending in movies?

I assume there are some “happy ending” movies that don’t end with a couple, but if there are I can’t think of any (or I haven’t seen them).

Also, why do all (or a majority) of movies have a love story as plot or sub-plot? Everything from action movies to comedies to horror movies show some kind of love interest. Why is that? Why don’t single people get a happy ending in Hollywood?