It's because of the caster effect. The pivot axis for the steering is tilted back just a bit, and the imaginary axis line contacts the pavement in front of the actual contact patch of the tire. If you hold a bicyle by the rear seat and tilt the bicycle to one side, a torque force turns the front wheel inwards.
So when you ride a bicycle, as the bicycle leans, the front wheel automatcially turns inwards, and will correct the lean. The front fork is bent forward a bit to reduce the corrective action. If you have a bicycle where you can turn the front wheel backwards, push and release it, it will almost stop before falling over.
In order to initiate and hold a lean, you need to apply a bit of opposite pressure to overcome the corrective force. At reasonably slow speeds, you can lean inwards, which causes the bicycle to lean outwards, self-correct, and then lean inwards because of the offset center of gravity.
On a motorcycle at high speeds (100mph or more), the gyroscopic forces resist any turning of the front wheel or leaning, and so much so that body leaning does virtually nothing. The amount of opposing force required to get a motorcycle to lean at high speeds is a lot. Also the corrective response is virtually gone. It takes almost as much inwards steering effort to straighten up as it take outwards steering effort to lean over.