Possibly because PIC16's are not really designed to do what you have in mind. Of 93 PIC16's that are listed on MicroCHIP's website, only 6 have self-write capability (which is what allows you to do what you trying to do). OTOH, of 155 PIC18's, 135 of them have self-write. AFAIK, ALL 24-bit core devices (PIC24F/H, dsPIC30 and dsPIC33) have this capability - which I have used on many occassions. You may want to take a look at this App note:
AN1095, Emulating Data EEPROM for PIC18 and PIC24 MCUs and dsPIC DSCs
http://ww1.microchip.com/downloads/e...tes/01095b.pdf
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