Income will surely change, but some of your loyal subjects will have to face the winter without food, as you are taking it to pay your growing armies and greedy wizards. Add patrolling armies that hunt down complaining farmers, killing their revolting kids, raping their daughters to keep the unruly in line and you have an additional bunch dead peasants. After a while you have fewer farmers to pay your taxes.
To sum up: Players dislike evil men and if they would raise taxes they would be evil, so they try to be nice, at least to their own people.
Therefore there are fewer players that would consider raising taxes. It is nice to have a community of morally correct people.
We count on the same moral standards to keep blood magic and necromancy from being widely used