Socks are portable
Socks are small and fit in most bags and purses which makes them the perfect project for knitting on the go.
They're easy to keep track of
It's so easy to pick up a sock and knit a couple of rows or even just a few stitches and then put it down again and not lose your place. This makes socks ideal for knitting in social situations, in the car (when you're not the driver), or in a waiting room.
Socks are (relatively) inexpensive
Socks are (usually) a one skein project, which makes them less of an investment than a multi-skein project. Sock yarns are available in many price points which means there is usually an affordable sock yarn for most knitters.
All feet need socks
Chances are if you have feet you will need socks at some point. Even in warmer climates people like to wear socks around the house, on chilly evenings, or when they travel to cooler climates. At some point in their lives, all feet need socks.
Learn new techniques
Socks provide a great opportunity to learn new techniques on a small scale. If you want to try a technique for the first time, socks are a great way to learn it, and figure out how you feel about it without making a sweater size commitment to the new technique.
Socks are worked in the round, so there are no purl rows to create the stockinette fabric. I personally don't mind purling, but I know some knitters would rather avoid it and socks are a great way to minimize purl rows.
Socks aren't instant (the average mid-calf single sock takes 10-15 hours to complete depending on pattern stitch, foot length, and gauge), but progress is quick enough that there are definitely satisfying moments: cuffs are quick, heel turns will always make you feel like you've done something slightly magical, and toes are pretty fast.
Socks are customizable
Sock patterns can be modified to accommodate almost any sensitivity or foot issue. I wrote a post about it here.
They make great gifts
Who doesn't love a pair of lovingly crafted, customized, hand knit socks? With personalized sock wrappers? You can find some gift wrappers for socks here.
Have I convinced you yet? I can keep going with all the reasons that socks are the perfect knitting project. Let me know why you think socks are the perfect knitting project.