You are assuming edible plants to have been readily available at high enough concentrations at the hunter gatherer stage of human evolution.

It is energetically more expensive to roam a large area to gather x calories than finding and killing an animal (herbivore) that did that for you - never mind the ability of the herbivore to release calories contained in leaves and grass (something humans cannot do)

Then in winter this ratio increases even more.

I don’t know if you’ve ever been to the African savannah, but early human evolution here was definitely not driven by the abundance of edible vegetation.