The reason is that replacements are very expensive. Years ago one was stolen from our primera whilst my wife was in a shop, and it cost £180 to replace it. You have to buy the whole unit. Nissan do no see fit to provide only the heated mirror.
I suspect that these thefts would not occur, were the mirror only to be made available a reasonable cost. They are in fact quite easy to replace separate from the complete unit.
If price was the only factor they would go to a scrappy for one. They are just bad b.....ds who have no intention of paying for anything when they can get some other sucker to shell out. It's only going to get worse because there is no retribution these days.
The mirror and backing plates are available, might not be OEM - the annoying thing is that in this case they are less than £9 each.
Most scrappies don't seem to sell on wing-mirrors, almost all of them offer to sell you new ones.