Quite a few enemies do roam. The Intellect Devourers on the beach. The Hook Horrors. The Zhentarim. The Minotaurs used to patrol an area of the Underdark, until the latest patch. The Owlbear and her cub mooch around a fair bit, although in a fixed pattern. Depending on how you enter, one of the adventurers in the crypt walks along a route, one of the paladins of Tyr is a wanderer, and most of the goblins never bleeding STOP moving.
Unless you mean roam *with complete freedom*, but that would be a nightmare, as you could find yourself up against the Gith with a level 1 party. The sections would have to be separated somehow (they're all in the same giant file at the moment, hence no mid-game loading).
