It's the same with workstations. Although they have plenty of room, but OEMs often install barely-capable CPU coolers that are noisy (rather than something like a large Noctua cooler that can keep most CPUs cool with barely any noise). Or another big annoyance: many OEMs (even reputable brands like Dell) install a PSU that is just enough for the load. If you want to replace the GPU by something that requires a bit more power, you'll end up replacing the PSU as well (assuming that you can get one that is compatible with their custom mainboards).
I'm not sure what the current technology is like as I haven't built a PC in a long time, but it used to be the case that efficiency would drop off quite rapidly if you underpower it. e.g. your fancy 600W 95% efficiency PSU would only be 60% efficiency at 200W power draw.
> OEMs often install barely-capable CPU coolers that are noisy (rather than something like a large Noctua cooler that can keep most CPUs cool with barely any noise).
My old AMD Athlon mainboard came with a very rattly and noise cooler for the chipset; I ended up replacing it with an after-market heat-sink that I got for €10. Pretty bad design IMO, but it saved ~€3 on the mainboard price.
A lot of the old Atom CPUs (Atom 330 etc.) had fanless CPUs, but because they were almost always paired with an old inefficient chipset they all came with a little cooler on the chipset. Ended up replacing a few of those, too.
This. I have got an old HP EliteDesk from work. It uses a standard SFF (not sure if it's the right term) PSU, but the motherboard connector is custom. Of course the fan makes a terrible noise. And even if I'm able to find a replacement PSU that would fit in the case (even fanless! - I'm happy with the integrated graphics), I also have to look for an MB connector adaptor.
FWIW I've taken apart many laptops over many generations and the general trend seems to be that OEMs always use a cooling solution that's just-so good enough, regardless of available space and laptop cost. When you give an OEM a 40 % reduction in power through a new CPU generation, you're giving them a choice: a) spend the same amount of money on a cooling solution that's as capable as before, so now you'll get a quiet and cool laptop or b) make the cooling solution 40 % smaller, making it cheaper, and keeping noise and heat roughly similar.