We used TV trays in the 1960s, but not every day, it was only when special shows or footballs games were on during dinner (no VCRs or DVRs or streaming; watch now or never).

I think the low-carb craze had the biggest influence. If you eat mostly carbs for a meal, you're hungry 2 hrs later and feel like you needed a snack. Curiously, we did have cookies & milk after school, but during the summer we'd pack our lunches after breakfast and explore and build forts in the woods. The only requirement was to be home at 6:00 pm for dinner. We were so busy we didn't think about or need to eat all day long.