I suspect that it's more about the geometry of best fitting people into the space. It allows the right amount of room for centerpieces and bread bowls and such. At least as far as the caterer and wedding planner are concerned, conversation is the last thing on their minds.

Those tables are intended for eight comfortably. They stretch to ten. You can do twelve, but everyone is gonna be cramped.

It's a size and shape that's so omnipresent one expects that it's some combination of geometrical efficiency, ability to pass/reach things, some flexibility in self-organizing conversational groups, and probably some other things such as not being too large to setup, teardown, and store.

Could be - making sure that everyone at the table always has a choice of conversation to listen to, instead of being trapped in one they have no interest in.

I doubt it's that explicit, though. Maybe it's just worked out better that way over the years, without anyone fully realizing the why?