My maternal grandmother knits very well -- I wear her work even now (as "retro" designs, granted). My paternal grandmother and mother both knit poorly and sew poorly. *g* In China a lot of clothes were handmade, of course, and my mother made some clothes for me up to my mid-teens because we weren't rich. So to answer one of your questions: it's entirely possible to be an adult who has had to sew clothes for their family for years and still be unable to turn out anything except amateurish assembled rectangles.** Just like it's possible to be an adult who had to cook for their family for years and still be a bad cook (I know many of those as well).

Practice is a big part of it, but to improve any skill you also have to want to get better, which (I suspect) implies an interest in the activity itself rather than just the result.

I'm about 500% more times interested in cooking than in sewing/knitting, so as an adult I've just decided not to do it. XD;;; I don't even darn socks or fix buttons that fell off. That part makes me feel like a terrible person, because I can in fact darn a sock or sew a button (even a buttonhole!), but if I decide I'm going to do it myself that just means I never get to wear the garment again, because I won't get around to it. Also, I never understood how to get the thread to run through a sewing machine, so there's that.

** That's my grandma. My mom can do a basic unlined A-line or H-line sundress. But office wear, or lined... forget it.