Why do Google+ profiles rank so highly for "site:Google.com"?

Because the "site:" operator does not rank on popularity or PR. I think it never has, and if it did, it was changed a long time ago. It merely shows you the indexed pages for a site. You can still gather a little info from it to see, for example, which pages are likely to appear in the secondary index / supplemental results ("In order to show you the most relevant results, we have omitted some entries"). Pages in index is somewhat of a vanity metric. It can be more insightful to look at the number of pages that send you at least 1 visitor a month.

If Google would rank "site:" on popularity people would probably (ab)use it for icky stuff like PageRank sculpting or figuring out how (internal and external) linking influences ranking, much like they did with popular search words /trends.

In the same vein "Link:" does not show all pages that link to a site and doesn't order them by link strength.

If you want more reliable information on links and page importance set up webmaster tools and analytics.

It might exclude the main http://google.com because you're doing a site search, and it assumes you've looked at the index? It might be missing most of the other pages specifically about Google because they have a lower ranking/less incoming links than the Google+ profiles listed in the search?