Perhaps the reason you never thought about it is that it isn’t in fact true.

The writer offers almost no evidence for his claim. And all sorts of reputable groups like this one give exactly the reverse advice.

I wrote a longer rebuttal and it’s here.

You should particularly note that during the time period 1900–1950 where the writer is saying we were poisoning ourselves with vegetable oils, Americans’ life expectancy actually increased by twenty years.