Plain vegetables are not palatable because they have very little nutritional value, like saw dust. If you were as rewarded for eating vegetables as you are for foods with actual nutritional value you would quickly be malnourished. If you ate a vegetable only meal you wouldn't even absorb the fat soluble vitamins from them because they don't have any fat.

In the wild where the immediate pressure is on getting enough calories and macronutrients while lack of micronutrients only cause problems in the long term, any human wasting energy collecting and digesting food that doesn't have calories and macronutrients (vegetables) would be quickly weeded out.

Look at the nutritional information of any vegetable for yourself. Fruits, nuts, legumes, grains, potatoes etc all come with the fiber, vitamins and minerals that misinformed people think can only come from vegetables while also being loaded with calories, protein and essential fatty acids all which are pretty much completely lacking in vegetables.