Not to dwell on a potentially sensitive topic, but one reason for empty seats may be to limit the weight of the ride vehicle.
When the vehicle leaves the loading platform, if it’s too heavy, instead of going ahead to the right, it’ll get diverted to the left to unload. I know because I’ve been in the vehicle when that has happened (it wasn’t me, I swear!). The riders are then put back in the loading area to be re-loaded with better weight distribution.
The CMs are good at keeping quiet about it and not making a big deal, but when I chatted with the CM she told me that if there’s multiple larger people in the vehicle, or even if there’s a vehicle that gets loaded with all adults and no kids or smaller people, then the ride operators are supposed to leave some seats open.