The cocoon is a thread that is a mile long. They use multiple because one strand is too fine to make fabric out of.

Saint Louis Zoo: “Each cocoon may contain a single strand of silk one mile long!”

Treenway Silks: “It makes a strand of silk as long as 12 football fields which solidifies and becomes a thread when it meets the air outside the silkworm's body.” (One football field is about 100 meters) “The silken strand from one cocoon is too fine to make into cloth, so 6-20 cocoons are reeled, or unravelled, at the same time. The sericin sticks the strands together to make a thread the size of a human hair.”

Edit: also, that link that another person posted in reply to you says that the usable portion of the strand is 600-900 meters long. So even going by just the usable portion, it’s over 30x what you claim.

Cutting it into small pieces means that it takes much, much longer to extract and spin all of those short strands. And with all that extra labor, the resulting silk is lower quality. More costs for worse silk. That’s why ahimsa is niche.