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.
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.