Because you haven't seen enough. These were from Crosstoberfest, last October. It was the first time I took my cyclocross bike off-road, and I had no idea what I was doing on it.
"Hmm. This bike weighs less than my mountain bike and that makes it easier to get over barriers."
The bike really wasn't dialed in for me yet. About the only thing I did to try to make it fit me better was to make the drops position a little lower and tilt the bars down to make the drops a more "comfortable" riding position. I also had way too much pressure in the tires and I was getting no traction on the off-camber stuff, so I fell a lot. Dirt on my shorts. Dirt on my knee warmers. Dirt and grass actually sticking out of the front shifter. At one point I had to stop and whack the shifters back into position because repeated impacts had them pointing inwards at too sharp an angle for me to work the brake. And I still thought I needed those to race 'cross.
And that riding position... By the end of the race, my back was killing me.
One of the cool things about that race was that while roadies are busy being serious about things and trying to have looks of quiet courage as they do ridiculous climbs or run around in the mud for half an hour, mountain bikers have been having fun since they started modifying Schwinn Excelsiors to be more fun to ride down the fire roads of Marin County. Crosstoberfest was put on by a mountain bike racing promoter, lots of mountain bikers came out, and they all cheered for my jersey. I got more support in that race than any before or since.
It's interesting to see the pictures from that race after spending the rest of the season on that bike. By the end of the season, I was a little bigger and a lot more efficient, and I had a much more comfortable position on the bike.
The pictures were taken by a woman named Amanda who thought she was photographing a couple of guys who had a similar pack position to me. Thanks, Amanda. Here's a link to her Picasa page.