Iirc VC as an industry loses money (or has poor returns), with the top 10 firms dominating the returns and some small number of startup firms posting great returns (iirc this is from a Jason lwmkin's post)
I suspect that what you're seeing is basically than the large firms are successful enough and funded enough so they don't need more money that they "negotiate" themselves 100% of the value. What can you bring one of the top-10 investors that they don't already have, such that they'd be willing to share the benefits with you?
So i think your index fund performs quite poorly.