Because socks do not lie to you about how much yarn you will need to complete them.
Socks have no sleeves to "ease" into armholes whose patterns came from someplace altogether different than the rest of the garment.
Socks have that simply amazing heel turn. There is nothing else remotely like it in all of knitting.
Socks, even if the toe is wonky, are generally covered by shoes and subject to lots of "oohs" and "aahs," which an ill-fitting, poorly-seamed sweater simply won't receive.
Socks are the closest thing knitters have to instant gratification.
Socks won't have you start over three times, with three different yarns, and three different patterns, trying your damnedest to make your son a sweater.
I think I shall knit a sock.