This would delay the start of the show, as they want everyone seated before they begin.
I'll say this as a general statement whenever you see show loaded with empty seats or even a ride vehicle with empty seats... It sometimes takes more time to make the effort to fill all the possible seats in a theater or ride vehicle that it is truly worth, especially if a show or ride has limited time for loading. Preshows can also complicate the matter. Do you let more people in who haven't seen part of the story to skip ahead to fill seats? While typically, we notice more empty seats in a wheelchair or disability viewing area, empty seats occur all the time and at time it is easier to keep trucking through than holding things up to fill to capacity.
Here are a few examples:
-The Walt Disney World Railroad has to run a strict time limit of loading at each station to ensure that trains (all independent of each other) keep moving around the track and don't have to back up which can be hard to apply the breaks on a train. Even if there are empty seat available, it is best to keep to the time in order to prevent any problems later. Carousel of Progress is similar in this nature as it has a time limit between each rotation.
-Attractions like Beauty & the Beast and Fantasmic! have set show times. Since it is unsure when guests with disabilities are going to arrive, the reserved areas must remain reserved until show time. At that point, the show much begin to stay on time with all shows following. Throttling guests to keep filling unused seats may delay that.
-Attractions like Haunted Mansion or Nemo, have ride vehicles back to back. Attempting to send a family to catch up with a vehicle before the end of the belt may result in a ride stop for everyone, best to let it go.
-Attractions like Dumbo or the Carousel may have empty spots, but by the time it takes to figure that out, grab a few extra people, get them situated and check any restraints if necessary, the ride probably could have completed a cycle.
-Last but not least, with preshows it depends if guests fill all the available space around them or if wheelchairs and ecvs need extra space. The only attraction I can think of that has bench seating with a preshow is Monsters Inc Laugh Floor. They have a target, but depending on if guests fill all the space properly can depend on if they filled the show well or if there was still room left for a few more. Wheelchairs or not.
Finally, some wheelchair viewing areas have seating split apart to accommodate a wheelchair or ECV. These split seats can also be hard to fill as you want to fill the next available guest, but you also want to make sure they don't' get split apart.