F/O Kite
In the old days due to various structural factors people could not make wings with less than a thickness chord ratio of about 8 or 10% (or even more) without them being too 'floppy'.
If you take one such 'thick' wing and bend it back (like the Lightning you mention) then the oncoming AIR sees effectively a much increased chord for the same thickness/spar depth - hence the spar is more streamlined and offers less drag.
Straight wings (if you can make them thin enough) avoid a lot of the stability and control as well as handling problems that are inevitably going to arise with sweepback.