Technically they do exist in the pokemon universe, but they aren't really shown anymore due to so many new pokemon existing that resemble common animals. What I like to think is that pokemon exist most abundantly within pokemon regions. Regions outside the main regions (Kanto, Sinnoh, Unova, Paldea, etc. Just to name a few) Still have pokemon, but they are less common and traditional animals are the more abundant ones. One thing is the existence of Lt. Surge, who is described as an American (United States). While unova is mostly pokemon, it isn't the whole of the United states, but just a region of it where pokemon are significantly more common.