One simplistic way to think about it would be that while random chance has a lot to do with whether a molecule gets to the vicinty of a receptor, once it’s vaguely in the neighbourhood it isn’t all just random luck getting into perfect binding position: chemistry is ultimately electromagnetic, and opposite charges attract by a real force, so the more positive parts that want to bind to negative parts etc., so the right parts of the receptor and molecule will be attracted accordingly until they bind.

Everything in physics is trying to find a local optimum, and there are real forces guiding them to that optimum.