Because most people would not build another computer if they had to buy another License.
This is not about me by the way, I have enough licenses to sink a battleship, but I do really struggle to believe that it's fair to be charged twice for doing the same thing, when no one has done any work to make my second machine work.
based the entire logic you've stated in this thread (which is faulty beyond reason)
if you bought 10 mac's, Apple should only be charging you for 1 copy of the OS.
so go request a discount on each.
Thats not how it works. LIcensing is a per device license. you agree to that when you purchased the device or license. Matter of fact, contract law. no excuses.
for each additional computer you are installing that OS on, you are depriving the company of further sales of that product.
These pieces of software aren't cheap to make. Windows has taken decades of continual development and advancement. Millions of Man Hours and I couldn't even imagine the sheer cost that has been spent on such.
in order for that to be doable by any company, they have an expectation that they can charge and set their terms on how you use their software. in Microsoft's case, that is a per device license (it's actually Apple's too). If Microsoft calculates that to even break even on development of the software they must sell X copies.
now what you're doing is trying to justify two things, violating the Terms and services (a contract) that is set between you, and Microsoft at time of purchase and two, Theft of digital property. You do not, under any moral, ethical or legal right own the physical code that exists within Windows. You are paying a license for the right to use that code. That is is.
So, if you install your Windows on 5 machines, but only paid for one, that is 4 additional licenses Microsoft is now not selling. That is 4 less sales. 4 less revenue sources. you might not think it a lot. it's only you after all, what's 4 additional licenses to Microsoft's big bank? well, what if millions of people did it? what if suddenly now Microsoft loses money on Windows? What happens to the thousands of employees of Microsoft whose livelihood depends on their work being sold?
The simple truth is, you are being cheap and are trying to justify to yourself why that acceptable to you, when simple fact of the matter is you are breaking contract law and in many places criminal law.