I have a theory on this and can speak from my experience.

The problem I think with NOT eating together is that it sews disharmony in the machinery of the household.

If everyone DOES eat together all the work related to it happens together as somewhat of a team. You do the thing once. Everyone is in a shared state of being. Specifically being satisfied in appetite and social interaction and ready to do there own thing after.

Otherwise Billy is hungry at a different time than Sandy and Sara wasn't hungry until it was time for bed. Dishes have to be re done for each person. Billy made a better dinner for himself and left none for his sister. Someone ate in the bedroom and left a glass of milk out over night and mom was angry in the morning...

I am rambling and maybe it's obvious but ultimately I don't think it's just the act of eating together I think it's the reduced friction and increased cohesion of the household that shared dinner causes.

To answer your question, Maybe family dinner forces you to be "doing it right" just by the act of having the family dinner.

I also wonder about whether there is a quality criterion there or is quantity all that's needed. For example if most dinners consist primarily of yelling at kids to sit down, eat vegetables, quit playing with food, just hurry up and swallow it, small bites, not that small, no poop talk at the table, inside voice, quit dropping your spoon, feet off the table, no kicking, don't annoy your sister, STOP SHOUTING.... Does that count?

Actually, almost definitely yes! Those things you mentioned, despite being friction areas, are actually loaded with all kinds of cultural and class norms, and the fact that you have parents who care enough and are around enough to be able to set those boundaries is a major selection criterion for success in other areas.

That's not to say you can't screw it up. If family dinners are spent in silence with the kids praying they get through the dinner without setting dad off again and watching him beat their mom to a pulp, or hoping that they don't get sexually abused later that night by one of their family members, etc. Then yes, maybe there are some quality issues, but I'd argue even in that extreme case, it might still be a positive indicator against the base cases of abuse without that element.

I was a paramedic. There is a massive fraction of society with upbringings like you wouldn't imagine. That's what you're up against. When people say they have a dysfunctional family, it's often pretty mundane. Especially compared to people in that class that didn't realize until they were an adult that things like getting raped by all of their mom's boyfriend's isn't a normal thing everyone deals with as soon as they start puberty.

That sounds outright terrible. Yes, it's not as bad as being beaten or raped (as mentioned by Enginerrrd in a sibling comment), but that's a really low bar.

I see that spending 30/45 minutes as a family is very important for communication. It helps understand daily life with its joys, issues, milestones. It helps bring peace in the family, and emotional support as well as practical planning. It helps to know each other. It's a no pressure situation, whoever wants to talk can talk, it's the best way to open up.

I've done it all my life, as a child and a parent, I have no idea why people would not want to enforce this.

This is a question of the causality between the family dinners and the socio-economic indicators of a kid. Just like most of similar questions, it is healthy not to assume causality until proven there is.

IMO, having regular family dinners is just one of many characteristics of stable and healthy families.