KTM Superduke High-speed Crash on Highway POV [Video]
It's not the wheelie itself, but the fact that the traffic is moving at high speed and it only takes a split second of insufficient awareness to run into other vehicles, run spilled bikers over and so on.
Even more, you don't have to be a racer to figure out that while on one wheel, steering is dramatically harder in case you have to swerve to avoid an obstacle. And if an emergency brake is needed, you're dead.
Well, the rider in this movie had a lot of luck in the end, especially for missing the guard rails. An impact in the solid steel structure would have most likely maimed him, given the alleged 140 km/h (87 mph) speed. And speed was great, judging by how long has the Superduke slid along the highway.
He got a thorough thrashing, however: you can tell this by his ripped leathers and gloves. The bike is a bit bumped, as well, but all in all, we might say he made it ok. For now.