Originally Posted by The Errant
It's there because the wonderful RTW AI is a hardcoded idiot that dosen't understand even basic economics.
When you notice a drain in your treasury you either disband your units or go on the warpath. Sooner or later you will turn a profit. Either from the loss of units in the conquest or by trough disbanding the units and focusing on the economic structures that bring in cash.
The AI does not understand how to do this. It does not disband units. Ever. It has no concept of bankrupcy. It's an all or nothing steamroller that hates the human players guts, and will not be pacified by alliances or gold. Sooner or later it's bound to attack you, if you share a border.
In order to keep the AI from plummeting into a massive debt where it wouldn't be any match for the human player, as it could build no new structures or train any new troops to defend itself from outside agression, all AI factions get scripted money bonuses based on the number of regions they control. For the smaller factions it means they have a chance to expand and build, and hopefully be a challenge in the future for the human player.
In the case of large factions, or huge factions like the Seleucids it means you have a monster with a virtually limitless supply of money, who hates you and will wear you down trough attrition unless you beat it first.
If it's any consolation to you MTW2 has a moddable AI so EB2 will be a lot better in this regard. It may not even need the cash bonuses at all. But that may just be wishful thinking on my part.