Because you'd have to have engineered the whole thing for that purpose right from the get go. In theory you can run the generators in reverse and push water up the hill into the basin. In practice this may not work for a multitude of reasons (priming, encasement, rotation reversal, cavitation, impeller and impeller housing design).

Also missing in vast majority of the dams is a lower reservoir. Pumping up the water from a river/canal below the dam would result in a dry river bed just below the dam rather quickly

*edit* spell