There seems to be a strange subconscious myth, in the US anyway, that you must have a "hot meal" every day or you are being nutritionally deprived somehow. I say subconscious because no one ever defends the point directly or explains what the benefit is.
This comes up a lot in political discussions of child hunger and school lunch programs. "X millions of children don't get even one hot meal a day!"
It's more about what cold meals stereotypically are--some processed cereal, two slices of wonderbread with some bread and ham in them--that drive that rhetoric. Sure, a cold corn and arugula quinoa salad is certainly much healthier than the vast majority of meals, hot or cold. But on average hot meals are likely slightly better.
If you can't heat your food up then don't pack something you need to heat up. If your food needs to be warm to eat and it doesn't go inside a Thermos then don't bring it or learn to eat it cold.
Life's hard. Deal with it.