They dont want to do that because they knew that if you had an npc server offline then there would be no point in renting a playerworld for a lot of people. They'd just go "Well we can get everything done offline"
Remember, back then everyone was like "OSHT WE NEED TO GET THIS SERVER ONLINE SO WE CAN FIX THE SCRIPTS AND SHIZZLE"
They wouldnt give you a free npc server to play around with offline.