It isn’t just the taste. The body shape and muscle structure of dairy and beef cattle are quite different as they have been selectively bred that way for many generations. If you took two cows the same weight, one of each, the beef cow will dress to much more beef of better quality and the dairy cow will produce much more milk and of a better quality. The two types are raised and fed differently as well.
If you are thinking that old dairy cows could become beef well somebody has beaten you to it: they do. But it isn’t prime rib or fillet, its tinned stew or soup, or hamburger.
