But that was not your original premise. "Many" games are not single core. Rather, a few old classics that refuse to die are.
Probably most games on that list don't use all cores. So yeah not "many" but "most". Thanks for the correction.
Per-thread performance really hasn't improved all that much since the days of, say, a 4.7 GHz 2600K. Core counts have gone up drastically, but that doesn't help your single-threaded game from 2007.