Because they decided that they'd learn to sew by making their kid a Halloween costume. And then they never sewed again, because all that ended up happening was broken thread, wobbly seams, wonky stretchy fabric and a kid who cried on Halloween when they had the lamest costume on the block. And lots and lots of swearing. And maybe a little banging of the head against the sewing machine.
P.S. If fabric stores were smarter, they wouldn't carry any of that awful fabric for Halloween costumes and encourage people NOT to start with the most difficult thing ever to sew. Because slippery knits? Fake fur? Shiny? NOT EASY. And then, THEN! they have the nerve to add a zipper to the costume. Yeah. I'd totally quit, too.