Pencils can be quickly swapped for different hardness. Swapping the lead grade in a holder is much more involved. Some artists may even find subtle differences between brands of pencils and may prefer to quickly swap.
Leadholders are almost invariably quite thick. The feel in hand is very different from the slim woodcase pencil.
Whenever I write by hand, I tend to prefer a fountain pen, a mechanical or a woodcase pencil. Writing with a Mitsubishi Hi-Uni or Tombow Mono 100 pencil, for those who can appreciate it, is a cheap luxury. Pencils are higly versatile writing instruments. I'm left handed and heavy handed and prefer H or 2H pencils to prevent smudging. Writers like Vladimir Nabokov praised the Eberhard Faber Blackwing (about 2B on a typical modern pencil scale, although these aren't standardized) for its smooth, almost pen-like glide.
American manufacturers can't compete in lowest price, but they might in best quality. The Germans and Japanese already do.