the tuning on guitars seems fairly symmetric… each string tuned to the 5th fret on the string above it… but why exactly is the 2nd string tuned to the 4th fret on the 3rd?
It certainly doesn’t have to be. There are many different tunings of guitars. The tuning you describe is just the most common (in the USofA.) I don’t know for certain, but I thing that that “1/2 step down” allows for the guitar to be played in more keys than if it were tuned with all strings at the prior string’s 5th fret. Try tuning the B and high E strings up 1 fret (1/2 step) and try playing some chords and you’ll see what I mean.
spirtle is right. Some tunings are designed to produce a chord when you lay a finger (barre) directly across the strings on the same fret. If you wish to play a song about Iraq, you may want to use the BAGDAD tuning.
All together on three.
one
two
three
Groan!!! 
The western music scale doesn’t have the same interval between all notes. The octave, a doubling or halving of pitch, is divided into 12 half tones which are all an equal interval to the next. Most notes are two half tones apart with the exceptions of B-C and E-F which are a half tone apart. That’s why there is no black key between them on a piano. That disruption in the scale is what makes different keys sound different. Play all the white keys from C to the next C and you have a major - happy - scale. Play all the white keys from A to A and you have a minor scale.
Anyhoo, if a guitar is tuned with all the stings being the same interval apart a couple of the strings would be sharped (or flatted) notes. No reason y ou can’t do that as Sritle said but the standard tuning for a six string guitar is EADGBE