Why does not every ninja have a KG?

[Majora]

Why is it different, if I'm not mistaken the heart pumps blood into the body therefor he should have several bloodines.

Yeah, but it is not Kakuzus blood?

 

[Rokusho..]

As i've said. Tsunade has not even shown any elemental ninjutsu, so my guess is that she can't use a KG at all. Orochimaru knows wind only, not enough to creat a KG either. Jirayia might have had a KG, but since SM>KG(well most of them anyway) he didn't show them in his battle with Pein.

If Jiraiya had one , then would be the time to put it to use :|

 

[Noble Armada]

By this logic, everyone should be born with a Sharingan, but not everyone can activate it.

It's just like hair color. If both your parents are blonde, there's still a chance you'll be a brunette. It doesn't automatically transfer.