The rubber on the interior car controls (basically the same as the one on the fob) tend to deteriorate much more quickly in cars owned by females. Females tend to use hand lotion a lot, whereas men tend not too. If a key fob's rubberized coating has deteriorated and there are no physical markings on the metal of the fob, then imo it is due to a lotion or oil of some sort. Abrasion from other objects can of course score and rip the rubber as well, but if it is just falling apart as in deteriorating and it is not kept with anything attached to it or in the pocket, it is most likely due to hand lotion/oil. Some people just naturally have oily sweaty hands which might also be the cause.
If anyone is in a brave mood you could always test it out by spreading hand lotion on their fob for a few days and see what happens! Let us know results.