Let's try a car analogy for a compatibilist position, as I understand it: there is car, and why does it move? Because it has an engine and wheels and other parts all arranged in a specific pattern. There is no separate "carness" that makes it move ("automobileness" if you will), it is the totality of its parts that makes it a car.

Will is the same, it is the totality of your identity which creates a process by which choices are made. This doesn't mean there is no such thing any more than the fact that a car is composed of identifiable parts means that no car exists, it is just not a basic indivisible thing.