Most angel investors need to decide whether to put up the money based on a few one-hour pitches and what they read in the press. Those are pretty bad sources of information if the biggest challenge of early-stage investing is judging the founders, because the founder is in "pitch mode."

YC alums in a position to invest can interact with the founders for months in very social and relaxed settings as peers. That's orders of magnitude more data than what you get in normal angel investing.