Given that human beings like to play with fire. You never hear of spontaneous combustion of livestock.
I did see the documentary a few years ago trying out the pig-in-a-blanket wicking theory (Horizon I think). Incidentally Nat Geo tried to replicate it but failed in their experiment - apparently they left the room door open and too much air got in so a proper conflagration ensued.
But, livestock don't wear clothes/covers, only a few have the right fat content to support "SC" (hence why pigs were chosen); being livestock they tend to live either outside or in well ventilated sheds; they don't get drunk and if they suffer from diseases that would make them stay unconscious through such an incident they'd be put down... I'm sure a number of other factors could be found here.
I can't find any instances where clothing or bedsheets etc were not involved in alleged cases of SHC. And the whole wicking process seems to rely on a closed room and clothing/coverings so oxygen doesn't get in too fast. And all too often there seems to be an ignition source involved on investigation - so hardly "spontaneous".