I honestly have no idea why we are so rare. But I have theory for it....most of us don't exactly understand people, and why they do what they do. We don't like to be judged so most of the time we keep to ourselves. I personally don't let many people into my personal life, and generally I won't carry on a conversation about my life with someone I'm not very close with. We also keep our friends for a VERY long time. I have had the same best friend for 14 years. So we honestly don't ever feel the need to add another person to our 'inner circle'. You probably meet more of us INTJs online because we are more comfortable where people don't see us, and we are 'protected' by a little screen. We can socialize better this way because we feel as if we can have more control of what happens. On the computer if we don't want to talk to someone we can just log off. But in real life we can't do that. We would literally have to stand there and pretend we care about a meaningless little chat.
The reason it seems as if we change from being an INTJ to another personality is because we can pretend to be something else very very well. We could easily be the chameleon of the human world. But most of us don't ever really care what others think so we are fine with being odd, or different from the normal person. And yes, many parents are not very fond of the idea that their child is anti-social. I know my parents aren't. they were not pleased when I was home on a friday night sitting on the couch watching Tv. So my guess would be that many children do change their personalities when they are called 'anti-social'. Just to please their parents.
Females are probably extremely rare because we are more susceptible to be criticized. Girls judge other girls very harshly. So many teenagers change their clothes, hair, and personality just to fit in. But INTJs that are really self-confident won't change for others even when we are catergorized as 'strange' or 'weird'. So that's basically my theory why us INTJs are so rare. I'm not saying this is 100% true. But I'm usually right, so I wouldn't doubt that it is.