Using significant progressive front brake lever force slows the bike dramatically, shifting weight onto the front wheel, lightening the rear wheel. All the braking available is at the front wheel. The rear wheel can do nothing to stop the bike once in the air, and will now lock with almost no brake pressure.
Passenger cars have a preset a brake balance.
The overall brake efficiency of a car dependent the driver's ability to modulate one pedal. A motorcycle rider must modulate a foot pedal and hand lever to get the best stopping performance. It is quite possible for an experienced rider to performed a controlled lock of the front wheel, where the bike rotates around the contact point and the read end leaves the ground. Locking the front wheel and skidding drops the motorcycle, or it is alternately possible to go over the top and have the motorcycle follow. This is a fairly popular maneuver among many younger, dumber riders.