it isn’t though!!! it’s because most relationships aren’t worth the effort. The “sweater curse” is actually most commonly called the “BOYFRIEND sweater curse.” Which=heteronormative, but the curse most often falls on a woman knitting a sweater for a boyfriend. Before she finishes the sweater, they break up - pop culture would have you believe it’s because the boyfriend freaks out do to the weirdness/clinginess of having a sweater made for you, but I think knitters are wiser than that.
It’s because after spending serious £££ on materials, and then HUNDREDS OF HOURS OF LABOR on the creation of the item, with every stitch a prayer of totally focused intent, creating a large display of technical skill - it is then gifted to a non-knitter who does NOT APPRECIATE the work/effort/skill/cost/TIME it took to make it, and in fact thinks you’re a bit weird and making a big deal out of a piece of clothing, and after they go “oh thanks” and shove your creation in the cupboard next to a sweater they got for £15 at an M&S sale, then they never wear your sweater because it’s too tight because when you asked them how their favorite sweaters usually fit they said “I ‘unno” and when you measured them for the fifth time and asked, rather tersely, if they had enough room in the chest, they said “I guess,” and then if pressed they say they don’t really like the sweater design, but then you point out that they were supposed to participate in helping you design it and they say they don’t really care about how things look, and when you say that you tried to match it to their other clothes so how can they hate it, then they say that honestly their mother still buys all their clothes because they hate going shopping, and that they hate all their other clothes too, well. That’s when a sensible knitter goes “Fuck this shit. And you know what? Fuck this man.”
This is what happens when someone posts in a knitting forum “Attack of the sweater curse!” - this is the usual story. It has a rigid plot. It is as old as myth.
That’s when you look at the time you spent and realize, “I could LITERALLY have written the first draft of a novel instead of doing this.” That’s when you go “I could have taken that £200 and bought myself a new wardrobe.” That’s when you go “I could have taken all that intent, all that willpower, all that creative force, and laid down some fucking witchcraft, all right?” That’s when you go “I basically spent 100 hours straight thinking about this bastard while making something amazing for him, and I have no evidence that he ever spent 10 hours of his life thinking about me.”
And “I could spend this time and energy and money in making myself an enormous, intricate heirloom silk shawl with just a touch of cashmere, in elvish twists and leafy lace in all the colors of the night, shot through with subtly glittering stars, warm in winter and cool and summer and light as a lover’s kiss on the shoulders, suitable for draping over my arms at weddings or wrapping myself in to watch the sea, a lace-knotted promise to myself that I will keep for my entire life and gift to my favorite granddaughter when I die, and she will wear it to keep alive my memory - but instead I have this sweater, and this fuckboy.”
The sweater curse is a lesson that the universe gives to a knitter at an important point in their life. It is a gift.
Knitting a sweater for a husband or wife generally doesn’t call down the curse, because the relationship is meant to be stronger than 4-ply.
(Although I say this, but I’ve taken over 5 years to finish a pair of mittens for my husband, because he casually asked me to do something customized with the cables, and I still can’t get the math to work on the right hand.)