Read the chronicle Seri.
While the surface read is "Quafe+ uses people as an ingredient," there is much more to it than the surface read.
It is about how megacorporations work. It is about Quafe acquiring and using a small business. Its about young star managers using the acquired business's product in a larger corporate project. Lastly, when it was found out the larger project was doing something unsavory, blamed it on the small business's prior leadership and destroyed their lives.
It is about greed and plausible deny-ability. Thrown into the story is metrics on how many die in a small capsuleer engagement or deadspace.
To answer your question: Why do Quafe need to use people to make (Quafe+)?
The same reasons people like pig Bacon and not turkey Bacon. The same reason it is preferable for capsuleer (and other) clones to be made out of human biomass versus cow biomass.