I think the reason whole fingers was picked over joints or phalanges is because when humans started counting stuff the probability of someone having missing a w

I think the reason whole fingers was picked over joints or phalanges is because when humans started counting stuff the probability of someone having missing a whole finger or two was higher than now (The life of a hunter-gatherer, accidents happen). When you are missing a finger you can still use your "stumps" to count in base-10, but your ability to count using joints or phalanges is impaired.