A republican form of government.

I thought that our elected representatives were supposed to issue a vote in proxy for governmental affairs at the level of their office. So why do our congressmen, both Senate and House, vote strictly on what is good for the nation? (Their level of government.) Why do they, instead, vote for what is good for their constituency? My understanding is that those we send to Washington are to look after the whole country, those we send to Phoenix, remember -- I'm in Arizona, are to look after the best interest of the entire state, and so forth for county and city government. If our politicians would follow that precept, which I believe is evident in our political structure, wouldn't government be less expensive, more effective, more reactive to problems that do arise, and more proactive to forstall problems that are not yet adversly effecting us? Wouldn't the nation thrive if we had politicians who looked after one indivisible nation instead of their individual states? Wouldn't the states thrive if they were under the umbrella of a nation that was operating as it should? Wouldn't the states thrive if we had politicians looking after the single state instead of the one county they represent? Instead we have politicians whose main concern is for themselves and their work is toward furthering their own carreers by bringing back benefits for their voting constituency at the expense of the greater population. On a personal level we call that theft. When someone breaks into your home and takes things that belong to you because having them will benefit him we call him a criminal.