Why doesn't a radar-equipped Tower say "radar contact"?
From the ATC Orders 3-1-9(b)(4):


  Unless otherwise authorized, tower radar displays are intended to be
  an aid to local controllers in meeting their responsibilities to the
  aircraft operating on the runways or within the surface area. They are
  not intended to provide radar benefits to pilots except for those
  accrued through a more efficient and effective local control position.
  In addition, local controllers at nonapproach control towers must
  devote the majority of their time to visually scanning the runways and
  local area; an assurance of continued positive radar identification
  could place distracting and operationally inefficient requirements
  upon the local controller. Therefore, since the requirements of para
  5−3−1, Application, cannot be assured, the radar functions prescribed
  above are not considered to be radar services and pilots should not be
  advised of being in “radar contact.”