FPSs are also generally confined on a map by map basis very seldom with a consistent world. Everything is prerendered and scripted on that map.

MMOs however, takes much more memory in the long run due to it's persistent world and needing to render more things at a time. You don't have NPCs in FPS matches for example, nor do you have constant events happening that isn't scripted into the maps background, for example planes flying overhead etc.

So while both are online games, MMOs are a different beast.