induced drag
And induced drag is pretty much proportional to the square of the lift coefficient and inversely proportional to effective aspect ratio. Lift coefficient with flaps may be about 2.0 whereas on retraction 0.5 is nearer the mark. Depending on loading of course. Effective aspect ratio is the square of wing span divided by [wing area x fiddle factor]. For a well designed passenger transport aircraft the fiddle factor is about 0.85 with flaps retracted and maybe 0.5->0.7 with flaps. So you may indeed want to have the same lift but by modifying the shape of the wing you change aspect ratio and by dangling bits you change the effective aspect ratio. The effect is more induced drag.