Because they (the legislature) are under no threat of removal unless they tick off enough of their constituents, they are looking out for themselves as much as the senators are. Pro politicians supporting pro politicians.
So, I (or a State) am (is) not entitled to the representative that I (they) feel is best qualified to represent my (their) interests?
Since, according to your position, neither the State nor the Federal Legislatures can be trusted to look out for our interests, why have them at all?