Not true. You can count knuckles on one hand up to 12, two hands up to 144, or 60 if you’re using fingers of one hand and knuckles of the other. Societies used to count knuckles, which is why duodecimal systems were a lot more popular back then. Way easier for doing fractions and divisions in your head as well.
You can count all kinds of things, but of the pieces of human anatomy, fingers are the most convenient because you can fold them while counting. Which is why they're also by far the most popular things to count worldwide, even if different societies use slightly different ways of doing so.
You can count to 12 that way, too, e.g. by folding the thumb first to indicate 1, and then unfolding it with the rest of the fingers folded when you get to 6. Or you can turn the fist upside down etc. Again, there are many techniques, but counting by folding or unfolding fingers is the simplest and the most obvious, which is why base-10 is the most common.
Decimal is a system best suited for written math.