Except it wasn't either sloppy or lazy. It had a different design goal than the PS4 [edit: oops meant PS4, had PS3 in there before the edit]. Extremely low acoustics to fit a potentially quiet living room to enable media playback without the intrusion of sounds from the device. To that end the design was brilliantly executed, marred by only one thing. The small fan used in the external PSU meant that quality control was exceedingly important. It wasn't quite up to what it should have been and some few people got external PSU's with noticeable sounds from its tiny fan.
The XBO-S follows the exact same design principles, except that they included the PSU in the case. And unlike many systems where the PSU is included, the PSU is cooled by cool air from outside the case.
Contrast this to the PS4 where the PSU is cooled by the hot air from the CPU cooler. However, the Slim PS4 design seems to address this, as it appears that the air cooling the Slim PS4's PSU is unpolluted by the hot air from the CPU cooler. That likely helps with the reduction in internal volume as you can use 2 smaller fans (lower in height) versus one larger fan. As the airflow required will be less for the PSU (cool air versus hot air) and presumably less for the SOC (assuming SOC is on a smaller process).
Regards,
SB