Because a government that can search any person at any time can falsely incriminate anyone, and motives for doing so are abundantly self-evident.
"During a routine anti-terrorism sweep, civil liberties activist John Doe was found to be in possession of methamphetamine, child pornography, explosive-making material, and pirated ABBA songs. He was immediately taken into custody and is being held at an undisclosed location for the public's safety..."
Right now we have an important check in the form of a search warrant. Before searching me, a law enforcement agent must demonstrate to a judge probable cause that I have committed, or will commit, a crime. It's not perfect, and there are notable loopholes, but at least there is some documentation and accountability.