Six-sided dice traditionally have opposite faces adding to a constant, and this tradition is carried over (unquestioningly, it seems) to other dice. Is there a good reason for it? Why wouldn't you instead seek to make opposite ‘hemispheres’ as equal as possible, ideally arranging that the center of gravity of the pips is the geometric center of the die? (This is not possible for D6; I haven't tested it for any other numbers.)
To prevent skill from being a factor in rolling
Dice have an even distribution of the numbers around the faces to prevent (or at least reduce) the ability to modify your chances of a result by skillfully rolling the die. Dice are intended to be entirely luck based, and depending on how the numbers are distributed around the faces of the die, could lead to skilled rolling (generally considered cheating).
It is for this reason that Spindowns are generally not allowed to be rolled anywhere that a random result is required (IE Tournaments, Organized Play, etc). For the same reason, d10s shouldn't be rolled to get an Even/Odd result as you can very easily force the desired result.