One can live on pure animal product alone and not need any supplements, yet one can't do the reverse. If you eat only plant food you must consume supplements.
There are plenty of nutrients found in meat and not found in plants.
B12 is the big supplement not found in plant foods, and the supplement was invented in 60s. So no true vegans could exist until then. Vegetarians existed, but they ate some animal products.
Plants also have no Vitamin A, only carotenoids which is difficult for our bodies to convert to Vitamin A.
The vitamin D found in plants is inferior to the vitamin D in meat. Most of us don't get enough sun for adequate vitamin D.
Vitamin K2 is also severely lacking in plants.
Heme iron is also only found in animal products. You need heme iron. For most people iron overload is not a worry, you need some genetic disposition to absorb extra iron.
We can't eat grass like a cow, but we can eat that cow.
You've made some claims without an examples. Can you give me an example of nutrients that are found in plants and not in animals?
Also the definition of an "essential nutrient" is a nutrient that we can't synthesise ourselves in our bodies. So no we can't make any essential nutrient because that's the very definition of the phrase.