Because they can is a good answer. Although a good XLR with gold pins is only slightly more expensive then some RCA's and considerabley less then the better ones. So this is not a good reason. The extra wire thing....well that's the manufacturers problem for not designing a balanced cable to begin with. Generally the shield is what is referred to here as the third wire, so any twin axial with shield can be used as a single ended cable or balanced. Discovery Cable has never charged one penny more for balanced cables and never will.
And for your second part, yes some electronics are not true balanced (phase correct and phase inverted) even though they have XLR's.
And for your second part, yes some electronics are not true balanced (phase correct and phase inverted) even though they have XLR's.