Question

why are some cocoons not boiled

Answer 1

THEY DO NOT BOIL THEM BECAUSE OF TWO REASONS :
1)THEY LET THEM GROW SO THAT THEY GET MORE EGGS.
2)SOME THINK THAT KILLING THEM FOR OUR USE IS NOT GOOD SO THEY JUST DO NOT KILL THEM......

Answer 2

Cocoons are placed in boiling water to dissolve the adhesive that glues one strand of silk to the layers below. This allows a single silk strand to be unrolled from thecocoon (usually about 20 strands are unwound together and this fiber bundle is then spun into a thread that is plied and subsequently woven).