Your mental idea of "desktop pc" probably doesn’t consider monitor, speakers, keyboard, touchpad, camera and most importantly battery. It isn’t mobile, wear risk of damage is limited. The case of a desktop pc is merely an afterthought, in a laptop it’s protection and interface, basically the thing. A desktop pc is usually also rather overpowered, since heat/energy demands and high component costs are of less concern, where laptops are often defined by compromise between physics, costs and tech at a certain time. Why would you bother upgrading an overpowered desktop CPU/RAM for slightly better energy efficiency, where in a laptop a „node size jump“, or even new ISA can mean double battery life, or less thermal throttling.