The bandwith usage is too great, and there are other constraints. (ie: only a certain number of peds can be kept track of at a time... so it'd be like 50 peds on 2 islands.)
this is not very true
an MMORPG uses 3kbps a sec
while CS and GTA seem to use 10kbps?
I dont understand how this can be when MMORPGs have more to handle then a FPS, maybe MMORPGs are more well coded.