Think about it, it makes sense. The metal chassis is just that, it has no functional use .... the motherboard is what makes the computer what it is.
Actually, no, it doesn't make sense at all—the only purpose of the policy is to make more money. Today, if you build your own PC, more than half the cost of the PC may be the cost of the operating system.
If you change the engine of a car, you aren't obligated to replace all the tires or the chassis.
In any case, although Microsoft will not say so publicly, it is much happier to sell an OEM license than it is to have people using cracked versions of the OS. As a matter of fact, offhand, I'm not sure I know anyone personally who is using a legal version of Windows on his computer, aside from a handful of non-expert users who are using whatever was installed on the machines they bought. But even machines with preinstalled, legitimate versions of Windows tend to get wiped after a few years, when something goes wrong; and then many computer shops or friendly geeks will install cracked versions. Usually this is because the user has no media from which to reinstall the original OS, or has restore media on the disk drives but they are broken or infected by malware and cannot be used to rebuild the OS.
As an example of how widespread illegal versions of software are, when I left a company I was working for some years ago, there was practically a fight among other employees to get my office PC, because it was probably the only PC in the building that had a full suite of legal software installed … nothing pirated, original disks and documentation (I have a thing about being legal). And that was in a corporate environment.