Yes, it is a bit of an odd choice, if that is what it is; to not have sims sit down for a longer time. Perhaps they were worried it might make them look unanimated, if not moving around. But they could do other things, while sitting, to give the appearance of lifelike behaviour.
I have seen that more often in restaurants lately. Perhaps there is a bug report at AHQ we could click Me Too on?
I just had a dinner party, of eight sims, mostly, sitting down to eat; only that one or two sims got up from their seat, standing by the table eating and talking, then sitting down again, and another getting up.