It's not up to me to justify anything, it is what it is.

What is: Our shoulders are designed to through objects, not hang from trees, our stomachs are highly acidic, at scavenger-like levels, we have a large small intestine for absorbing macros and at the same time a very limited capacity to extract energy from fibrous plant foods due to a relatively smaller large intestine (i.e., convert them to fatty acids, and amino acids to a lesser extent, like cows do), we also have many small fat cells, a pattern that is common in carnivores and supports a metabolism where dietary fat is our biggest energy supply.

I don't know where all the fiber was available throughout the ice age, for sure it is important when consuming processed food (where the natural fiber has been stripped), refined carbs, or added sugars, because it acts somewhat as an antidote, but the idea that a juicy steak should be paired with any amount of fiber has no basis whatsoever.