I guess because our parents went through a war and a depression, and remembered when food was not always abundant, they had a horror of wasting food. I remember being made to stand out in the kitchen 'until you've cleared your plate'. My tactic was to wait a while and either feed it to the fat black lab, or chuck it through the kitchen window where said fat lab would find and scoff it. I got the devil of a spanking one day when, once again faced with my refusal to eat, Dad said 'remember the starving children in India" to which I replied 'if they want it they can have it'.....however it resolved me never to fight with children about food. If they only like mashed potatoes and peas, well I'm happy to cook em every night, at least that's two vegetables.I figured eventually they'd try something else.
As for me, I still would not eat beetroot or brussels sprouts, not if you paid me a million dollars.