Senator vs Secretary

Several prominent politicians have given up their Senator status in exchange for positions as Secretary in Obama's Cabinet. I do not understand this as I view it a step down. As a Senator, you are serving the people of your state directly. As a Cabinet Secretary, you are serving the President of the US. Firstly, since I assume most politicians are ego driven, I would have thought being a Senator a more independent and prestigious position. Secondly being in public service I would have thought serving the people directly would be better than serving an individual, even the PotUS. Finally, as a political career move, what have past Secretaries risen to become?