It's a valid question in this age of Di2 capable, 10-, 11-, and 12-speed mountain bikes. I'll attempt to put some words to the issue here though they represent my feelings alone. I don't promise your satisfaction in my answers or reasoning.
For me singlespeed all started as I got sick of breaking Trek Fuel frames and maintaining rear derailleur cables in the dusty Front Range of Colorado. I got a Redline Monocog for Christmas from my wife one year long ago and quickly grew to love it's simplicity. There was nothing to adjust and minimal maintenance besides the occasional tightening of the chain and some lube. I just got to hop on and ride. When I moved to Boulder from the flatland triathlon scene of St. Louis, MO I tried to keep the climbing at bay. I stayed to the east, getting promptly thrashed by the hills anytime I ventured west. But something unexpected happened when I started riding that heavy 26" Monocog. I got faster. I became a better climber. The better I climbed, the more I began to love mountain biking. I ventured into longer rides in more difficult terrain. Probably a year later I ditched my front suspension for a rigid fork and haven't looked back since. To be frank, the lack of front suspension is a far bigger detriment to my speed on the local XC and Endurance scene then the lack of gears ever has been.
So what logical reason is there for riding and racing a singlespeed, especially down the 2700 mile Tour Divide? Well there are a few. First it's lighter by a couple pounds. My rig was one of the lightest out there despite me carrying more weight in gear than I did on my 2009 run. Second it's weather resistant. In 2009 the weather was pretty horrendous by comparison to 2016 and the mud was out of control. The singlespeed trucks right on through most of the time without any major issue. A straight chainline and some steel drivetrain components stand up to conditions that put a pretty serious hurt on geared bikes. My drivetrain this year went from Banff to Mexico without needing anything but a few shots with a hose and chain lube. Too bad I can't say the same about PF92 bottom bracket bearings. Third a singlespeed is cheap. I'm not sponsored in any serious way and pay for the vast majority of my gear and parts. Cheap drivetrain components are nice when you put serious miles on your gear training and racing. Most of my steel cogs are a decade old, the chainrings last thousands of miles, and chains are replaced for $30 or less.
People's biggest misconception (I think) is that a singlespeed is just so much slower than a geared bike in the vast majority of terrain. With proper training I'm not sure that's actually true and looking at Strava segments of quite a few pieces of the Divide seems to at least support my theory. I turned in the 3rd fastest all time segment (that's uploaded to Strava...) for the final 105 miles of Tour Divide in terrain that should punish a spun out single speeder pretty severely. While most geared riders train to optimize power in a preferred cadence range and then gear their bikes to keep themselves there, I chose a gear and trained my body to produce power over a wider range of cadences. I can make reasonable power for long time periods from 25 rpms all the way up to 110 rpms. Would I be faster I could stay at 85 rpms all the time? Maybe but I'm not totally convinced of that. My Tour Divide moving average speed was 11 mph, not a whole lot different than Mike Hall's. He still kicked my butt but probably not because I was on a singlespeed.
At the end of the day though, none of these what-ifs, how-comes, or whys even matter.
I just flat out love riding my singlespeed. It excites me. I'm passionate about it. Every time I throw my leg over my singlespeed and mash down on the pedals, the direct drivetrain connection feels so good.
Long ago I gave up any hope of being a "professional" athlete. Giving up that fantasy gave me the freedom to do what I want, ride what I want, and race what I want. (Again) this year singlespeeders in the Tour Divide have shown that they're more than capable of being competitive, no matter what the conventional wisdom might say. And with that maybe the why doesn't matter. Or maybe those who ride one gear are just so passionate about riding their chosen bike that it overcomes any obstacles the single gear brings. Either way I'm not likely to give up my singlespeed any time soon.