A few years back I read a brilliant article with the same title as this post. A friend of mine said the exact same phrase a couple of weeks ago and this instantly popped in my head. Did some digging and finally found it. Finally your questions are answered!

You could read the whole article with statistical analysis here.

A summary of the article:

(Statistical figures based on 1998)

A question that practically every male has asked himself at one point or another in his life.

Why don't I have a girlfriend?

The standards: First, the potential girlfriend must be approximately my age—let's say 21 plus or minus three or four years. Second, the girl must be beautiful (and I use that term all-encompassingly to refer to both inner and outer beauty). Third, she must also be reasonably intelligent.

Number of people on Earth (in 1998): 5 592 830 000

…who are female: 2 941 118 000

…in "developed" countries: 605 601 000

…currently (in 2000) aged 18 to 25: 65 399 083

…who are beautiful: 1 487 838
(I will say that for a girl to be considered really beautiful to me, she should fall at least two standard deviations above the norm.)

…and intelligent: 236 053
(Let's assume that I will settle for someone a mere one standard deviation above the normal)
 
…and not already committed: 118 027

…and also might like me: 18 726
(a girl would go out with someone if and only if they were at least one standard deviation above her idea of average)

At first glance, a datable population of 18 726 may not seem like such a low number, but consider this: assuming I were to go on a blind date with a new girl about my age every week, I would have to date for 3493 weeks before I found one of the 18 726. That's very nearly 67 years!