The float switch can fail just like every other component on the vehicle, ESPECIALLY if you or the previous owner has been using water in the washer pump reservoir instead of actual washer fluid (washer fluid won't corrode all the little metal parts). Might also be a bad gasket on the harness for the switch, I've never dug into it before but was just recently reading up on it. Remove the front bumper cover and the switch should be accessible.
Can you reproduce the problem in neutral? Take a picture of the dash if so.