I think if we apply:

1. Occam's Razor and

2. the Mediocrity Principal,

I think it's pretty clear that the answer is

3. Velociraptors.

So if they're unintelligent, they will find each other?

What @SkotlandSkye said. =] Circumstances keep people from finding each other. Other circumstances keep people from each other.

1. Distance
2. Fear of striking up conversation in the first place

Distance and there are a lot of people in this world.

Hell, I thought at the time my best friend and I were perfect for each other, but she was married to a guy she hated, and I... Well, I was single. Still am. I hate being single.

The Adjustment Bureau at work.

Usually it's due to eyes being closed.

1. Geography
2. Time
3. Surface reasons -- appearances, perceived wealth/lifestyle, actual wealth/lifestyle

For example, the perfect person may be in India or China whilst you are in the USA and you will never meet them. Or, the person may be right in your city but your paths never cross or they get hit by a bus and cease to exist in your timeline. It could also be that you consider yourself in a different socio-economic class than the person and therefore never give them a second glance or, conversely, don't approach them out of fear of rejection. Finally, the perfect person may be a redhead....and you don't think gingers are really humans....

Location, location, location.

Dave Alvin did a great song - Abilene - kinda sums it all up for a lot of people I think.
. Then too it depends on what one is looking for, or looking at all.
The flip side to being single for anyone is that there are reasons why.
May be them....may be you....May be a combination of the two......which, by the way, is another great song.

Life's a bitch.

Distance and/or time and search engine limitations

Law of averages. Bad timing. Just because. Or they're blind to the fact that they're actually right in front of each other but can't see it.

Because they live in different countries and don't have access to online dating ?

Because there are 7 billion+ people on the planet.

Well its not that simple to find each other. There is too much space as there is too much time, for destiny to put both parties in the same coordinates.....

Probability. It is possible to toss a coin 29 times and have it come up heads every time, just not very probable. Assuming there are a dozen perfect partners for you out there, the probability of finding them is about the same. One in world population divided by 12. And before someone says the population is half male and half female so that doubles the odds, the non desired gender is still there standing in the way so that doesn't change a thing.

Law of averages?

Location... opportunity... common interests. Check this out...

1. Im divorced. Shes divorced.
2. I attend the same function
3. I meet a beautiful lady. Her.
4. We talk about our same interests
5. I explained me to her.
6. We are both are planners and understand projected flow. Contractor Costs and permits.
7. we are both attractive and well groomed.
8. I own my home... shes in the process of buying hers.
9. Were both in our 50's but look younger.
10. We're in great shape and compliment each other.
    This is how i met my 3rd wife... lol. We've changed since then...lol. but its cool.

They are both imaginary ? Is this a trick question lol?

Yeh think people have said it all .. Time .. distance ... the random nature of life, numbers.

Shit Happens?

Physical attraction.

Opposing cultures

maybe for the same reasons we haven't found extraterrestrial life? It's a BIG universe.