Sometimes it's a matter of personal determination as well.
Yes, maybe one adult out of a 100,000 determines to improve their handwriting. But people usually have a more mature form of the handwriting they first learned in elementary school. I'm pretty sure that is where my father learned to form his letters, some of which were different than what later became the standard style. I will say that he always wrote in a more determined than casual way.
While going through some family items we came across a framed piece of caligraphy done by a relative from the middle 1800's. It was a tour-de-force of handwriting. Yet it must have also been a school assignment.