A number of examples. One example is that carpets might be fitted afterwards. Because the air tightness design in the first place is so poor, they resort to sealing between floor and skirting and other such bodges. Carpet fitters hate such sealing as it makes it harder to get a pleasing finish. So they rip out the seals.

There's a lot of folksy wisdom in the UK building trade about natural ventilation (which isn't really true). Many builders will deliberately expose small gaps because they have not been properly educated. Having gaps gives poor ventilation - it becomes dependent on ambient pressure differentials which are difficult to control.

All of these are excuses for poor design and workmanship in the first place.

This might seem like more work for minimum wage labourers on site, but it's less work for highly paid designers, so it costs less. But costing is a difficult thing to sum because it's a complex supply chain with opportunities for efficiency all over. It's just that the volume builders (in particular) have a conservative interest in keeping the status quo, and carrying on without rocking the boat.