*edit: Seems YC participated in the follow-on round after the batch ended, so I imagine some extra due diligence goes into that.
This applies to investors at any company. At some point you have decided to believe in the founders and executive team, rather than deal with daily operations and micromanaging your portfolio
The personal network is often more used by founders, who provide introductions to investors. You have to be social to be a founder, which is really why networks are more noticeable therein.