The problem is linoleic acid and our overconsumption of it. It seems to cause way more oxidative stress during metabolism, to which the brain is more sensitive. Plus it also seems to adversely affect metabolism of other kinds of fats. And it plasticizes during cooking.
The idea being in the past the only linoleic acid we would be getting was from whatever seeds we consumed naturally. With the advent of industry it's now a 20B+ business. It was hard for humans to consume so much seed oil in the past.
