This is because, in the absence of friction, there are no forces acting on the bicycle to slow it down. Friction is the force that opposes the motion of an object when it is in contact with another surface. When there is no friction, the object will continue to move at the same speed and in the same direction until it encounters another force.
In the case of a bicycle coasting on a level surface, the only forces acting on the bicycle are gravity and the force of the wind. Gravity is pulling the bicycle down, but the force of the wind is pushing the bicycle forward. Since these two forces are equal and opposite, they cancel each other out and the bicycle continues to coast at a constant speed.
In reality, there is always some friction, even if it is very small. This means that a bicycle will eventually slow down and stop if it is not pedaled. However, if the friction is very small, the bicycle will coast for a very long time before it comes to a stop.
