waiving right to jury trial

     "The Trial of all Crimes, except in cases of Impeachment; shall be by jury..."  No one actually has the right to "waive" the presence of a jury when being tried for a crime.  This supposedly only happens when someone pleads guilty, and a trial is considered unnecessary. There should be a trial, anyway, and with a jury. The jury should be present to hear the plea or confession. The trial would be short, so why the problem?