Because the function of the Senate is to advise and consent. The nomination is entirely in the purview of the Executive Branch.
The term "Advise and Consent" is legalize which describes one of two situations, either where the executive branch signs a law previously approved of by the legislative branch or where the legislative branch approves of a nomination previously presented by the executive branch.
They "advise" the President to sign a law they have passed or they "consent" to a nomination made by the President.

The term "Advise and Consent" is legalize which describes one of two situations, either where the executive branch signs a law previously approved of by the legislative branch or where the legislative branch approves of a nomination previously presented by the executive branch.
They "advise" the President to sign a law they have passed or they "consent" to a nomination made by the President.