Because it's a beautiful thing! I have had a nice geared road bike and a MTB for years, but last year bought a fixed gear for fun. Now I ride the fixed gear much more than either of the other ones, although it is by far the cheapest bike I own. (Ebay, the next fixie I buy will be a lot better). I find riding the fixed a better experience. I have limited time to ride (who doesn't?!) and a fixed really gets in good miles. I always feel like I've had a good workout after I come back from a ride. I like the way your legs control the ride, not the gears. If it's an incline, pedal hard, if it's a decline, spin fast. It's just a great way to ride. I didn't really believe the hype before I bought mine, but now I tell everyone they should try it. I also find my constant pedaling carries over into my rides on my other bikes, and I have gotten faster on them also.
My daily rides are solo, but I do the occasional charity rides, etc. and find I pass many, many geared bikes. You get mucho respect on organized rides. I've been told by people I'm passing comments such as "fixed, you're a monster!", "50 miles on a fixed gear, that's crazy!", "I felt good about my pace uphill until you passed me on a fixed!", "That was wild, you got some fast legs!! (after seeing me spin more than 170 RPM going downhill), and many others. My fixie only cost me about $200, but it also gets as many, if not more, looks than carbon wonder bikes. I don't ride it for the attention factor, but thought I would mention it.
I do find that as much as I like fixed gear riding, I do ride with a front brake, and do not do crazy messenger stuff like skid stopping, riding no brakes, etc. I ride mine like a road bike, just fixed. I do use my legs to slow my rear wheel, but use a front brake to help stop. I'm not into the semi-hippy messenger/track scene, but view mine just a road bike, but a little different.
In all, I love it, and my dream bike has changed from a Colnago carbon w/Campy to a custom Independent Fabrications fixie. I really think you should try it if you can, your riding may never be the same!