I get this too. What helps is to set the dining table by clicking on it and adding place settings. Once a dining table has place settings, Sims will all sit there. It works on outdoor tables too. I can't remember if it was added as a feature with a pack though, or if it's an option as part of the base game. It joined my game a couple of years ago if I remember correctly.
It can be a little annoying as you have to keep tidying the table but they will sit together.
It just annoys me when it's somewhere where that's not possible, I add camping seats when travelling but they still look for a pool to sit in to eat instead. Also they won't sit together at festivals.
