I think it probably stems from the days when knitting was less a hobby, and more of a necessity to keep the family in socks and whatnot. So a bit of a cultural/social hangover from those days, likely with a feminist aspect too (women's time/skills not being highly valued). I certainly observed that in my grandmothers, and to some extent in my mother. I have no problem knitting for myself, though I do enjoy making items for people I care about. I am fussy about who is 'knitworthy' however!