It re-uses existing systems (gimballing engines, computers) to effect the recovery. They do add some stuff (legs, RCS, and now fins) but those have nothing like

It re-uses existing systems (gimballing engines, computers) to effect the recovery. They do add some stuff (legs, RCS, and now fins) but those have nothing like the mass and aero penalties from wings. It also minimizes development cost, since it's a modification of the existing expendable stage and can be tested during revenue flights, rather than development of a whole new vehicle. Hovering a rocket, btw, is not that hard these days. Look up the Lunar Lander Challenge and/or Masten, Armadillo, Unreasonable Rocket, Morpheus, Mighty Eagle. Big and tall rockets are also easier to hover, given their large moment arm. Think balancing a broom on your hand vs a pencil. (Not that an F9-1 can hover--it's too powerful and too light--but it's a similar operation to do a controlled slowdown.)