Maybe because when people get their first dishwasher, they are single and it takes days to get a full load? When food hardens overnight they work a lot less well.

Somehow I've never been able to convince my wife to stop pre-rinsing.

How often do you clean the filter in the sump?

Mine has a line that runs to my garbage disposal, so normally no special cleaning is needed.

Most dishwashers are designed to drain into the disposal, but they usually still have some kind of filter going to the sump before that line. Maybe you have some extraordinary model which truly has no sump filter, but otherwise you should still regularly examine and clean out your sump filter even if your dishwasher drains to the disposal.

The water in the dishwasher is being reused by the pump and sprayer while in cycle. The sump filter ensures no particles larger than the sprayer heads are being sucked up by the pump and pushed through to the sprayer jets, as tough food particles could damage the pump or get clogged into the sprayer heads.

Every week or so; takes a few minutes. You'll need to do this even if you 'pre-rinse', btw.

Don't most dishwashers do a rinse cycle first before adding the detergent?