Simple reason - constant war mongering isn't something politicians will be supported for because the price for that is going on the people. That is, if politici

Simple reason - constant war mongering isn't something politicians will be supported for because the price for that is going on the people. That is, if politicians have to answer to anyone (thus democracy). Dictatorships on the other hand don't care about squeezing their own populations to the bone for their war mongering. Well, until the point there is an uprising of course and they are burned down by the angry mob.