[bjt]

The VC has access to a lot more than the "idea" from a pitching company. A lot of time and effort goes into researching a market opportunity and iterating on the idea to refine it. You learn which aspects of the idea, though they sounded sexy, turn out to be dead ends. Getting that information for free could give the VC's company a significant head start.

[Kalium]

Yup. This is why you need to have execution, instead of just an idea. You need to bring to the pitch something that indicates that you are better-suited to pursue this opportunity than the VC is.