This definitely feels like an overgeneralization. For what it’s worth, there are plenty of people who don’t say "itadakimasu" at restaurants but would say it when they’re invited to someone’s home as a guest, for example. More people, however, seem to say "gochisousama deshita" (thank you for the food) when leaving, whether at a restaurant or someone’s home.
Some people do say "itadakimasu" out of habit even when they’re eating alone. They may have grown up with stricter manners around meals, to the point where it’s simply baked into their routine. But it’s far from universal. Dramas and other shows may make it seem more universal than it actually is, partly because saying it out loud is more performative and works better on screen.
Some people do say "itadakimasu" out of habit even when they’re eating alone. They may have grown up with stricter manners around meals, to the point where it’s simply baked into their routine. But it’s far from universal. Dramas and other shows may make it seem more universal than it actually is, partly because saying it out loud is more performative and works better on screen.