why are people only attracted to few mates/partners?

Social conditioning.

they say a large part of it is pheromones and that we tend to "smell"/pick mates that have a compatible immune system- of all things.

yeap subversed by fear

humans ego indeed demonic vanity

Fate? Yeah you could say it's that.

Regardless there's pretty much zero choice in the matter.

cross out the fatties and incompatible people, about 1/10 maybe 1/7 workout, looks matter ladies, get with it, get in shape

Good question and deep. There are lots of questions to ponder. 7 billion on the planet and we wonder why we can't find the one. If you grow up in a small town do you pick one cause you might not get that chance to explore elsewhere? It's complicated like some women say but with as many choices there are on the planet, finding a look and personality that fits your genere could be difficult. It's like winning the lottery. Maybe that is the reason some cheat. They are with the person they don't want to be with so finding someone else is on the agenda while in that relationship. Finding a life long mate is never easy but when you do, you will feel the emotions you as us human's need....Love. There is no other ultimate...Peace.

Explain, demonic vanity and social conditioning? Oh gosh i have a few smelly friends who to others smell great! And i cant be a meter within them even as just friends. Some wreak the sex odor and it stinks, through. And i just want to run. I cant smell my partner and if its this primal its weird. So our immune systems subconciously know their matched? Then i see friends happily with fat abusive slobs, and these are academically smart ladies.