Look Ma, no hands!
Look Ma, no feet!
Look Ma, no teeth!
The point is, you cannot ride a bike without touching at least one more point than your saddle. When you ride without hands, that second point is your feet. Lose the pedals from under your feet, and disaster is unavoidable. You won't be able to do anything about it. You won't be able to grab the bar, you won't be able to brake (the brake levers are on the bar), and you won't be able to steer.
If you happen to be heading into a moat, you'll go into the moat.
If you happen to be heading for a wall, you'll go into the wall.
If you happen to be heading at a truck, you'll go into the truck.
You are just a passive passenger on your way to disaster.
When you ride without hands, your pedals are a single point of failure. Thus, you are risking your life for absolutely no gain.
Now, road racers click their feet into their pedals. This allows them to pull the pedal up at the back stroke, but it also makes their connection to their pedals much more failsafe than when you ride without this feature. If you do not have this extra safe connection to your pedals, don't ride without hands.
So, the best possible answer to your question would be: Because riders have wised up and learned to avoid this needlessly dangerous behavior. I don't know whether that's actually the case, though.
Ps: I learned this the hard way, including four weeks on crutches. Even a quarter century later, this accident still scores as my worst accident ever. Please learn from my experience instead of making your own.