Why does the keratinocytes do not produce melanin?

Mammalian keratinocytes make up the bulk of the epithelium, undergo keratinisation, and form the skin's dead superficial layer. These superficial keratinized cells in the lowest layer of the epidermis (i.e. the basal layer) are continually replaced by cells originating from mitotic cells.

Melanocytes are located in the basal layer and do not keratinize; however, they can produce pigments of melanin. Melanin is stored in melanosomes, the small granules present in skin.

Keratinocytes activate melanocyte functions such as dendritogenesis., proliferation, differentiation, and melanogenesis.

That is why keratinocytes do not produce melanin but they support  melanocyte in the production of melanin.

b - the respond is innate and varies over time.

good luck!

it would be rhyolitic hope this