For this reason: more upgradable
Apple wants to sell you a new system every couple of years, they don't want you to upgrade. They want you to throw it away the system completely and replace it (and they claim they are environmentally sustainable, btw) when in the PC world you would replace RAM, CPU, GPU, motherboard, maybe the SSD, but surely you can keep the power supply for decades, keep cooling fans, not talking about the chassis that could last 50 years because it's a just metal box.
They are a rip off these products, and I don't understand people that buys them.
BTW even when Apple did produce towers like Mac Pro they did so in a purposely non-standard way, such that you couldn't, for example, replace the PSU with a standard ATX one, the only standard components were really RAM, HDD, CD-ROM drive and in Intel models the CPU, and of course PCI expansion boards (sort of, because to work GPUs needed to have an Apple specific firmware in the old PowerPC models). You couldn't for example upgrade the motherboard, if you wanted to change the CPU to a new generation you also needed to buy a entirely new Mac with a new chassis, new PSU, etc even if what you had were perfectly fine.