Because I don't treat my phone like a piece of fine art. It get tossed in my backpack on my dirt bike, bumps around in a pouch while off-roading in my UTV, sits out in the back yard around the pool, rattles around on the dashboard of my boat (where it often gets splashed), goes in the lake with me to take pictures/videos, etc. My phones don't go from home to an office cubicle to Starbucks and back - they get treated pretty rough.
I've used a LifeProof case on my last 2 iPhones, and one will be going on the 6+ as soon as they come out with it. I don't care about aesthetics and don't consider a phone a status symbol to be shown off....I just want it to work and keep working. I was certainly glad my phone wasn't "naked" the day I accidentally dumped it out of the side pocket of an ice chest and it spent 2 minutes in 12 feet of water at the bottom of the lake. If I had been enjoying the tactile and aesthetic beauty of a "naked" iPhone, it would have been toast. Instead, it still works perfectly. Just for good measure, I shot a few underwater videos with it later in the day.
So I do understand why people buy cases, and why they're sometimes necessary. I'll gladly accept the tradeoff in (negligible) weight and bulk in return for the needed (for me) protection. I even go for redundancy and purchase AppleCare+ for my phones.