I think these days a lot is also because of online check in so they've never actually arrived at the airport.
That seems to become a popular theory here all of a sudden as an answer to the OPs question but think about it: it really doesn't make much, if any, sense.
It really shouldn't come as a surprise to know that at most if not all airlines the staff know (or can find out) who has checked in on line.
Somebody who checks in on line and then doesn't arrive at the airport and doesn't drop bags off is not going to cause a flight to go late...they either turn up and board the flight or not...it's of almost no consequence either way to the airline.
As a result there's often pretty much zero reason for the gate staff to start making PA's for somebody who checked in on-line but then might be stuck on the M25, had a domestic emergency and decided not to travel or maybe have not bothered getting out of bed...
Gate staff might put more effort into finding somebody who has checked in at the airport ...that probably would mean announcements.
They will put effort into finding anybody who has checked bags, so again there will be announcements...but in parallel efforts will be being made by other members of the ground team to get those passengers bags off the aircraft ASAP to try and ensure the flight goes on time.
.... of course at this point it's maybe it's worth a health warning that don't think, as some do, that having checked bags will always stop the flight going without you...