No, that's not necessarily the reason. The directory is built by volunteers, true, but that's irrelevant to the licensees (who are mostly commercial entities.) As licensee, Google has the right to make modifications to the directory (such as advertisements) if they wish.
Could it be that google doesn't see enough value in advertising in its directory to bother? Seems a bit wild, but what else could it be. It doesn't seem likely that it's part of their "do no evil" strategy.
Always good to hear that people actually use the data! But you may want to look at the "source" (i.e., dmoz.org) instead, as it always has the most recent data. (Unless you want to see the sites ordered by PageRank, of course; but you can always click the green ball at the bottom of the DMOZ page to go to G.)