I think you're missing the point entirely. Yes, there's a chance that one of the people who cold emails them is good - but really, what are the odds?

With introductions, the referrer stakes his or her reputation on the referee. Make too many bad recommendations, and that cuts out a bad lead. The most successful organizations I know recruit through internal contacts - and it just makes sense from a VC's perspective to do the same.

It's about the best allocation of time vs good finds, rather than just finding every good opportunity.