I get asked a variation of that question frequently. Sometimes it's my dear husband tentatively posing it when I've ripped something out after starting it three times and thrown the mess of yarn across the room. Sometimes, I'm asked that question while knitting in a waiting room or while my daughter is in her ballet class. Most non-knitters don't know that a really good sock yarn is not inexpensive so they aren't even aware that a comment such as "You know you can buy a whole bag of socks at Walmart for that cheap?" is way off base. Ha! If only I could get a skein of Happy Feet or Socks That Rock for the price of a bag of Walmart socks!
So why? For me it's much simpler and more complicated than a couple bullet points. But in the end, the reason I bother with the hassles that go into finding the perfect pattern for the perfect yarn, or getting gauge, or learning a new cast-on, or finally finishing a WIP is just this: "Because I can!"
This is one of the new techniques I recently learned. Arm knitting! It was loosely based on Simply Maggie's Arm Knitted Infinity Cowl. My daughter says it looks like a hammock and it is a little bulky for down south, but it was fun.