Anything that can be skimmed, stolen, lost, damaged, etc. doesn't work. My wife's piercings are ones she doesn't take out. She's still managed to lose at least 4 in the past 10 years that I know of.
An implant would work in theory, until it's skimmed - then what? Surgery to remove it and implant a new one? If it can be rewritten through the skin, what stops it from being hacked?
Then there's the fact that electronics degrade. No matter what medium, they'll wear out eventually. What happens if a person can't transfer their key due to money constraints? What happens when it's transferred without their knowledge? It's fraught with problems that may not seem likely generally, but when magnified to the scope of billions, it becomes a recurring problem.