There was a preview a while ago (6 month or longer) GSX will make pax sitting on their seats... dont know whats about this "feature", i think its still missing due to performance issues and/or the zombie looking of People in MSFS in general.
We don't have any performance issues; we just listen to users that keep asking for other things in the meantime, some of them were promised way before seated passengers, like multiple audio channels.
And people in MSFS don't look like next-gen games (and even next-gen games with budgets 1000x GSX, like Starfield, don't look much better, especially secondary characters), because the MSFS graphic engine lacks crucial features like morph targets that are required to do more natural facial expressions, because it IS a flight simulator in the end.
If those features ever arrive in a future version, we might then be even more careful with performance issues, since we would need to show dozens if not hundreds of passengers at once. It's the usual issue: you can't complain of both the "zombie looking" AND being worried about performance as well: they ARE surprisingly light on fps BOTH because we know how optimize things, but also because they don't look like main characters in a recent adventure game where characters are the main focus.