Is this because their funds are exhausted and they wouldn't be able to invest in new startups anyway? Or do they want you to find their general partners on Linkedin and pitch directly to them?
[1] https://www.blueyard.com/
Is it because they want to throttle the number of pitches they get? That would work, but wouldn't it also filter out the best and most promising startups, which will easily find a more approachable VC, and leave only the more desperate founders?
Economists, techno-utopians, and some VC's may talk big about how VC's provide an essential service in our late-stage capitalist economy - but that does not mean that VC's are a public service, under some sort of obligation to answer the phone and listen to whoever wants their attention.