Like many of us here, post-war rationing was still in force when my parents were very young, and even if it hadn't been, their parents had firmly implanted in them the idea that food absolutely could not be wasted once bought. It wasn't just profligate, it was a sin.
While my father was not as strict as my grandfather, my brother and I would simply not be allowed to leave the table until we had 'had a good go' at everything on the plate. And the compost heap was the standard repository for leftovers that weren't suited to the fridge.
I used to think I was living under some kind of despotic tyranny; today it seems eminently civilised. My wife, daughter and I are only able to eat dinner together on the weekend. Fortunately, my daughter is of the 'scoff it all and ask for seconds' type.
I grew up to adulthood (1950's / 1960's) under a similar regimen, but in our postwar American national and specific family / local sociocultural contexts the background wasn't quite the same.
Both sides of my family had been or started out as purely farm folk dating back only 1 generation on one side and 2 generations on the other side. Both sides had survived the depredations of the Great Depression. By the time of my childhood both sides still generated a lot of their foodstuffs from their own land and labor (though my parents' generation had shifted to blue and white collar jobs), and both sides were accustomed to meals incorporating a wide variety of vegetables and grains along with meat. Some dinners were vegetables only, with at least 4 different items.
In our household my brother and I were expected and required to accept what was put in front of us and clean our plates before being allowed to leave the table. On those occasions when he or I resisted, we were given one or both of two rationales for having to eat everything. The first, analogous to your experience, had to do with past hardship, thrift, lack of easy alternatives, and / or the occasional guilt trip of considering all the people who didn't have enough to eat. The second rationale - apparently derived from our parents' farm upbringings - was simply that the vegetables (etc.) were good for us nutritionally (whether we liked them or not), and we were going to accept them just as we had to swallow the even more odious medicines of the era.
Our extended family (living in 5 adjacent homes) grew a large garden every summer, and food waste from all 5 households was put into a substantial compost heap daily. Collecting and delivering any leftovers / scraps to the compost heap was the standard post-meal task alternative to washing and / or drying the dishes.
My elementary school was blessed with a creative cafeteria manager well-trained in nutrition and her staff of good down-home cooks (largely comprised of ladies from my family's church and my benignly imperious great aunt). There was no brown-bagging or other lunch alternatives. Every kid in the school received a solid 'square' meal of meat, at least 2 vegetables, a fresh salad or fruit item, and milk. Once everyone was served, we could go back for seconds for as long as the food lasted.
Looking back, I have to say we ate well, to our tummies' content, and probably "better" than many kids do today.
It was during this elementary school period that I first encountered (typically suburbanized) kids who wouldn't clean their plates as I dutifully did. I didn't think much of this strange behavior, because it garnered me additional helpings of my favorite vegetables.
By the time of my high school years I had a number of friends whose family and personal dietary habits weren't anywhere near as solid as my own. I didn't think much of the differences at the time and just accepted it on a "different strokes for different folks" basis.
Having been forcibly habituated to square meals thankfully accepted and completely consumed "or else" in early childhood, I had no problems eating decently all the way through my undergraduate college days.