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.
I agree Sam may be in an especially good position to make guesses about the future of companies, but that doesn't explain where the investment money comes from.
Also, to be an accredited investor, don't you need a net worth of like $1 million? So having 50k lying around doesn't cut it. Although there might be exceptions to that rule... I'm definitely no expert on the subject.
"income exceeding $200,000 in each of the two most recent years or joint income with a spouse exceeding $300,000 for those years and a reasonable expectation of the same income level in the current year"
And it is not like you have to submit to an audit to the company before sending them a check -- all you have to do is sign something that says you are accredited. That way you will have a harder time suing the company claiming you were mis-led or whatever.
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.