I wouldn't exactly call TV dinners "healthy". In many ways it was much more heavily processed fatty carbs than what passes for microwave food today. It was loaded with salt and sugar.

The main difference is that TV dinners were designed to be heated in an oven rather than a microwave, back when microwaves were less common.

My impression (as someone who was alive back then) is that the early TV dinners often weren't that different from something you'd cook yourself, but the industry has spend half a century optimising them into cheap chemical slop engineered for palatability.

Also, not all ready meals are crap. You can buy premium frozen meals from restaurant suppliers but they aren't cheap.