The last house we lived in---a moderately expensive and well constructed house, compared to many of the others we looked at---didn't have electrical boxes behind wall- and ceiling-mounted light fixtures. The romex was just directly connected to the fixture and the fixtures were secured with drywall anchors.
Electrical boxes are low cost during construction and address a real risk; electrical connections are where wiring is most likely to break and potentially cause heat and sparks. Electrical boxes are an effective mitigation to that risk, and really should have been included in construction. Retrofitting boxes in may be a lot of labor though. In bulk, single gang new work boxes are less than $1/box, and don't take much time to put in (two nails + a little extra time while wiring to pull the wire through the holes). Plenty of other places to save a little money and a little time.
For heavy fixtures, like large chandeliers and ceiling fans, appropriate boxes also help with bearing the weight; preventing the fixtures from pulling out of drywall and being suspended only by the wiring.
The thing is, in the absence of someone (like an inspector) specifically checking, you can assume that anything that isn't immediately visible uses the cheapest possible materials and the fastest, shoddiest installation.
In my case, it may have been as simple as, "we didn't order enough junction boxes and we're running late as it is; just skip 'em".
Disastrous?