I want the aztecs money. Why can't I just annex them and get all of it lol? 6 wars just for money gets old.
multiple reasons combined, some are more or less manipulaple some are not:
1. money have worth, in peace negotiations based on YEARLY income of country.
2. maximum war score is 100% for war, combined with 1 giving basic maximum for war
3. every year's(month's) end they will get more money, so they will replenish some money between and during wars giving false impression they actually had far more money in first place...
4. point 2. can be modified by good Casus Belli - Tribal Feud, Tribal conquest, trade War CB make you able to demand 2-4 times more gold per negotiation than possbile with most other CBs. They give 50-75% less cost for demand war reparation peace demand..
5. getting ALL country money by annexing is non-sense, especially so when waging colonial war - see how history worked with Cortez and Aztecs:
He got far more gold by holding Emperor as hostage and making Aztecs themsleves to collect the gold, after they killed him and looted the capitol, most of the gold was hidden away (at least tried to) from conquistadors - unless they surprised the natives(found the hideouts) they took less gold by force then they got by ransom - gold in both cases having more of spiritual/symbolic value to natives..
Generally, country that is completly overtaken by their enemies doesnt really like to give them national treasures
[they got plenty time, war was pretty slow in that times]
If only from spite..