Yikes...that just makes my stomach hurt watching!  I am very impressed with those who can do that, though.

The why part is easy.  You have street bikes that stall trying to climb a curb.  You have dual purpose bikes that walk over the telephone poles used to border parking lots.  You have off roaders who ride mule trails.

Then you have those who ride trials.  The do not go fast, but they can ride their bikes over any obstacle -- including seven foot walls.

I started riding dirt in '74 with my DT-175.  In '86 I bought the first TLR200 sold by Gardena Honda, at a discount.  The object was to be able to go places that stymied the Enduro.  The Enduro was actually about 75% street/25% dirt.  The Reflex was 95% dirt with the 5% barely making it street legal.  Forks could twist nearly 90 degrees left/right.  Weight under 200 pounds.

Before I could develop skill with the bike I got married and life sort of interferred with my biking.  Now, at 58, I'm no longer willing (or able) to ride off-road like I used to.

Modern trials and indoor trials is the most difficult of all motorcycle disciplines. Let’s face it, given the proper motorcycle, any motorcyclist could get around a racetrack, be it a road course or dirt track. Anybody can get around on a scrambles, or TT course, or motocross track, or a drag strip. An enduro would be tougher, but most would probably  make good progress.  I am not saying anyone can win or even do well, what I mean is you can ride and complete the course and stay alive. I suspect precious few motorcyclists would attempt a single trials section, let alone, score well or clean it. Todays trials riders are truly supermen.