Then why not get rid of the Senate entirely?
It’s just a more expensive version of the House.
Because allowing legislation to be approved by a single house is far more dangerous to liberty than requiring that it pass two separate houses (Washington’s admonition to Jefferson still holds true today).
And because in the U.S. Senate (i) members are elected statewide (so politicians can’t gerrymander to gain advantage) and (ii) each state has equal suffrage, so California can’t overpower the rest of the states as its population increases.
Having state legislatures elect Senators is not what made the Senate necessary or special at the Founding. Perhaps if state legislators were true representatives of thinking voters it would be acceptable for them to elect U.S. Senators, but that clearly is not the case nowadays. After you fix state legislators so that they act as representatives and not as politicians, come back to me with the proposal to have them elect U.S. Senators, and I promise that I would consider it.