Because waiting 6 in-game hours for a meal is bad enough without your Sim wandering off to the bathroom three times.
The is a popular mod for The Sims 4 by creator sims4me (also known as Bienchen) that prevents Sims from constantly standing up and wandering while dining. Restaurant Sit Tweak
There are also specific addons for Seasons (no more holiday toasts) and Werewolves (no more pack gifts at the table). Because waiting 6 in-game hours for a meal