Except agents of the State used to resort to even more trickery and outright threats and actual violence to get confessions out of people, including confessions from innocent people. They still do this, but Miranda has curbed some of the more egregious violations of 5th amendment rights.
Granted, but Miranda doesn't tell them they can't obtain confessions in less than honest ways. Similar, but not the same. We still have the right to STFU. Always have.