Here's why:

* DNA is not a protein: DNA is a nucleic acid, a different type of macromolecule. Pepsin specifically targets peptide bonds in proteins and won't affect DNA.

* DNA extraction techniques aim to preserve DNA: The goal of DNA extraction is to isolate pure DNA, not to degrade it. Using pepsin would destroy cellular proteins, but it wouldn't help in obtaining DNA.

Instead of pepsin, DNA extraction methods typically use:

* Detergents: To break down cell membranes and release the DNA.

* Enzymes like proteinase K: To degrade proteins, but these are specifically chosen to not damage DNA.

* Salts and buffers: To precipitate DNA and remove impurities.

Therefore, using pepsin in DNA extraction would be counterproductive and lead to the loss of DNA.