The mindset was to compact it as small as possible, and use the heat sink for some additional function, like board stiffening (or reuse an old existing one to save on tooling), and the ME and mechanical designer on the project didn't have an EE that knew the art, at least in this area. Note that the big (400volt) electrolytic is laid down flat, so it is a manual operation to mount, was well the heat sink hardware, and many other parts on the photoed assembly. Remember the goal is to all ways make it as small as possible, using the fewest parts, fewest circuit card layers (1layer is what I see in most consumer junk), and it's a SMPS, which is supposed to run super cool, it comes down to how many reviews are done with the right people who have the experience of seeing field failures, and learning what not to do, as well other mentors that have their gray beard experience, and hopefully one or two clever people on the team with real good pattern recognition skills that see bad design as they are that gifted.
My critique of this design is for high volume, it has too much manual labor. It looks like they built it with 100% all human pick and place assembly. I guess that saves on all that expensive equipment.
AND the vendor's only needs it to last 4-5 years, who the heck keeps any hardware longer then that (I do, but I'm frugal)