In pre-WW2 times, society ate what was “in season”. Nuts were picked in the fall and ate in the winter. By spring/lent time, nuts had gone bad. There was no global trade industry to get nuts shipped from S.America to US grocery stores.
Peanut butter wasn’t invented until WW2, for soldiers. Eating nuts/nut oils outside of fall/winter isn’t natural or historical. Or available.
If humans went back to eating what is “in season” they’d be healthier. In the hot summer months, it’s healthier to avoid oily foods because the more you weigh, the hotter you’ll get. And fats slow you down, which wasn’t helpful when the farm work needed to get done. And what foods are plentiful in hot months? Fruits. 0% oil. 0 proteins. Pure energy.
Sure, eggs were always available on a farm, but those are relatively low in oil. Mostly protein. And all protein in lent is forbidden, except fish. Fish does have oils, but it’s high omega 3s and fish oil doesn’t have the concupiscent effect on the body that animal oils do.