All the other points already raised aside, it's not in the interests of any of the major established stellar players to allow a new group to form that could challenge its power. It's not impossible that such an organization did try to form but was bought out by CHOAM, or offended the Guild, or got murdered by the Emperor's Sardukar, or got manipulated and outplayed by the Bene Geserit.

Also remember that this is a universe where FTL communication happens by couriers delivering messages on (Guild) ships. What are the bankers going to do if the Guild decides to cancel its own debts?