that's the obvious reply, but it fails for two reasons:

1) SERPs are not fixed; they vary from month to month, and even from day to day as new sites and pages appear and are added to the mix. This is akin to the houses on a street changing order frequently, and ruins the analogy

2) SERPs differ between terms - you and I might be adjacent for "cheap widgets" but for "budget widgets" the SERP might look entirely different. This is akin to shops having a million doors, each on different streets, and ruins the analogy.

PPC advertising on said search pages fixes the analogy. As do link directories and, erm, front pages of link sharing sites such as HN (if submitted within the proper timeframe)