A reason why VCs prefer college grads [though I would say that's more of a trend than the norm]....or, like you said, VCs becoming increasingly more popular for recent college grads.
While founding companies themselves, or being a part of many early teams can be taken as a prerequisite to FOUND/START a VC, working in VCs need not necessarily need such a background.
It's important to remember that ANY experience has a downside too--the baggage [aka mental blocks] associated with that line of experience. Unless the individual has the capability & inclination the relearn, such mental blocks act as inhibiting factors.
And this is where fresh college grads would score.....they [or at least most of them] come with a clean slate....NO baggage of any sorts They are impressionable & get to learn faster....what they are given to learn--meaning positive or negative ways of working--depends on the seniors of course.