This is because "supply" is often used in the context of providing something to someone. For example:
* "The company supplies its customers with high-quality products."
Here, "supplies" is acting as a verb, and the preposition "with" is understood to be present.
The sentence could be rephrased as:
* "The company provides its customers with high-quality products."
In this version, the preposition "with" is explicitly stated.