Because we live in a democratice representative republic rather then a series of autonomous states. We give money into a common pool that is ostensibly put to work where it is most needed (not saying this always happens, but that's the idea). This is most often less densely populated agricultural states.
Personally, I dont know that spending is the issue overall. But there are definitely areas that could due with some cuts. However I dont know that I would want to cut welfare, medicare, etc without a study to look at how said cuts would effect people. My opinion is that cuts should be made where they will do the least harm to the populace.