The thing is, it isn't so common as you think, it's just that you aren't used to seeing it so it sticks out to you. There are a thousand couples that pass by that you never notice because they are natural in your mind. Some stick out and it may seem like there are a bunch of them. Some cases:
-Mixed race couples
-Age Difference couples
-Attractiveness difference couples (works both ways I might add, I've seen some lady Jabba's with some sharp looking dudes.)
Now even with it being uncommon you have to think, there are a reason these things happen. It may be attributed to any hundred thousand of things. Maybe they get treated better, maybe it's real love, maybe he/she has a gun. Who knows. Let people be happy and mind your own business