It's mainly because the interiors were created some time ago, when it was not common to see passengers on trains. We are aware that people have noticed this and perhaps in the fullness of time it will get rectified. It's not a high priority though, you should be driving the train :-)
Passengers not a high priority? Don't let Sadiq Khan hear you say that.
I mean, it's not like we stare at them all day on those little CCTV screens to check they're minding the gap properly and standing clear of the doors for fun, is it?
On a side note, I was mildly miffed that the Just Trains S7+1 Advanced stock doesn't appear to have removeable passengers. It seems really iffy driving into the depot at the end of a run, with a train full of passengers. TS has the scope within scenarios to disembark the whole train, and to start empty - it's such a shame JT didn't make use of it.
I'm guessing it's the same on the S8 Advanced too?
In some ways, it's better that the D78 has no passengers, and just looks a bit empty (they're all at home watching the tennis or something) rather than having to pre-warn the depot foreman to get the kettle on, because we're heading in to ECMDT with another flashmob of party-goers, looking for a good time in the canteen.