why do companies charge double for xlr cables?

XLR requires one more wire. Price varies on type of XLR connector. I have a 1m pair in house with $230 in just the connectors.

Only if your components are differentially balanced (DB)designs will XLR cables do much for you. That said, DB defines hi-end. Finally, we are seeing more and more manufacturers using DB.

XLR connectors are probably twice as expensive. The wire has an extra conductor and is shielded. It doesn't need to be fully balanced to improve contacts and reduce noise from EM/RF.

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.

Discovery cable is fully balance as well and they will re terminate any of their cables to xlr for $100 They sound great to boot. Joseph runs a fair business- seemingly rare these days. I recommend him and his cabling!