In new construction people are installing those microwave vents that literally do nothing except blow the air out the top of the microwave. It’s not totally worthless but it’s next to worthless.
We use them all the time and if they're not vented properly, we don't get occupancy. The HVAC designer or Engineer on the job wouldn't have it any other way.
Yes and no, while I suppose you could technically attach a vent to the top half of the microwave and route it outside the upper vents are absolutely not designed for that and you would have the most awkward ugly installation that blocks your upper cabinet doors. So yes the reason for these units is 100% penny pinching and cheap construction but better construction alone won’t fix the problems with these microwaves.
"you would have the most awkward ugly installation that blocks your upper cabinet doors"
No, you route it through the cabinet and if you don't want it to look ugly then you build a chase around it. I usually don't since the chase takes up more room, and no one really cares as it's in the cabinet.
We’re talking about two different kinds of microwaves. There are the ones you describe where the vent is inside the upper cabinet, those are designed to be vented outside. Then there’s the microwaves where the vent is in front of the cabinet doors, runs along the whole length of the microwave and blows up at the outside of the upper cabinet doors. There is nowhere you could even route it because anything on top of the microwave to capture it would physically block the doors from opening.