Question

why are silk and nylon fibres soft

Answer 1

silk is the soft due to the silk worm high amount of agdgeigre in there cocoon

Answer 2

Unlike its cousins cotton, linen and wool–fundamentally coarse, short fiber fabrics–silk is made entirely from long strands of protein. ... The result is a type of cloth that is smooth all the way down to the molecular level, naturally soft and conveniently light.

I think the same kind of situation holds true for nylon fibres too,friend. Hope this helps you...