Many animal activists do not encourage sericulture. Why?

The rearing of silkworms to obtain silk is known as sericulture.  During the rearing of silk, the caterpillars start secreting the silk fibre around itself in the pupa stage. This caterpillar further secretes fibre & itself get covered under the thread. This is the cocoon stage. Cocoon has to break this fibre to come into the world but culturists need fibres to make silk fabric so they place it in hot water which kills the cocoon and culturist get fibres. For obtaining 1 kg of silk, 5500 silkworms are killed. Animal activists are against this process, as it involves the killing of silkworms on a large scale.  Because of this reason, animal rights activists are not in the favour of rearing of silk.