I believe the answer is in the virtue of Predictibility. Pre-cut packs of fabric come from a single designer/manufacturer. and from a single line, or theme. This guarantees that the colors will harmonize nicely.
The set I bought was the Benartex Fossil Fern collection, and I was amazed at the control over tone and hue, so much so, that the neutrals, pastels, briights, and dull colors like mud and slate grey are all amazingly coordinated.
Being a natural Scrappy quilter, I am sensitive to clashing, both tolerating it and avoiding it most of the time. Pre-cuts save a lot of grief for beginners, and folks who prefer careful well-matched color schemes.