Primarily because I don't like making my economic systems as intricate as Sonereal's and so, as a compromise, I set up a few guiding principles:
-Increases in taxation had an adverse effect on growth and vice versa
-High debt caused crowding out and hurt growth
-Large militaries sucked up resources that could be used more efficiently by the private sector and thus hurt growth
-Often people and institutions lacked the money necessary for independent development of hospitals, roads, schools, etc. so regular investment in these was a good idea
-Ideas spread after a few years, so every year the various technological fields received a stimulus from abroad
There are exceptions to all of these rules in reality (persistent low taxation can lead to inflation, for example, and crowding out can be mitigated if the debt is used to fund infrastructure that generates enough economic activity to offset the decreased money supply), but I felt a few guiding rules was better than putting in all variables. I want to run a game, not get a migraine.
Furthermore, since not everyone sees eye to eye on economic policy, a simplistic system is best. An overly complex system would lead to me being accused of bias by players whose personal ideologies don't line up with the game's rules.