Mine was neat and well formed as a child and onward, but then I think two factors had something to do with that: one was that I was precocious in drawing and sketching well, so it would seem that I had good fine motor skills because of my ability for art, which inherently needs good control.
The second factor was that my school pounded cursive into us as if it was as important a lesson as mathematics. Seriously, almost as soon as we learned our letters at all, we were started in with these cards full of printed cursive that we were made to copy for an entire page, like writing lines Bart Simpson style.
There were cards with something that looked like the infinity symbol upended and flowingly repeated, and we had to duplicate it. Because I already had an eye for "drawing" what I saw and doing it well, it was easy for me to copy these cursive cards and exercise that skill until the movements were part of my brain.
I went on to actually find writing well to be a pleasurable pursuit I took pride in, almost like a special interest very close in nature to drawing too.
I suspect artistic people might more often have flowing, good handwriting as an extension of their ability to draw, though obviously this won't apply to every artist.
Lately in my older years I'm having mental and physical health problems, my hands are tired and stiff, and my handwriting has become a scrawl, but I attribute that to my immediate problems.