Why do all airliners have life jackets?
The plane would be lighter.


Not by much.


  It cannot be hard to dedicate different planes to different routes.


This adds to the logistics difficulties in scheduling the airframes. It's MUCH easier if all the airframes can be used on all the routes.


  Returning the life jackets each time the plane has to fly over water.


This involves extra down-time/logistics before the airframe can fly the next trip.

You've used the phrase, "large bodies of water"; think about if the airplane needed to ditch in a "small body of water". It would be preferrable that all people had life jackets and thus a chance to live.