The World Domain is very big and it is filled with abundant resources. In reality, although the people of the four great clans have been collecting resources around the city for hundreds of years, there are still many places in the area of one hundred thousand kilometres around the city that are still completely unexplored.
Even without the help of people who came to the World Domain in order to use the Tower Of Ascension, the people of four great clans can collect enough resources for themselves because there is no shortage of resources in the World Domain.
Moreover, the people of the four great clans can't use the Tower of Ascension like normal people and go to Utopia. This is another restriction that the Laws of World Domain had placed on them.