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).
