Because if they did that, people would complain about how they have to jump through this or that hoop and it's some kind of artificial barrier to enter the market that is at the behest of the big name companies like Dell, HP, etc. And of course if they didn't offer it at all, it's the same basic arguments. They're doing it on behalf of large OEMs who don't want any competition, they want to force people into buying more expensive copies of Windows, you name a crackpot theory, someone will claim it to be the unvarnished truth.
I'll just build a system, install an OEM copy as I always have. Then buy it from myself for a dollar just as always.
That way I am fully in compliance with the EULA.
The only way I could see you having a prayer is if you do computer building professionally and are incorporated in whatever state you live in (assuming you live in the US). Then used corporate funds for all the hardware and software purchases then selling the system to yourself and paying for it with personal money. However, the taxes you'd pay as a corporate entity would far and away surpass any savings on this OEM version of Windows vs just buying the retail version. Plus you could potentially open yourself up to other legal issues for giving yourself, effectively the CEO of your company, a sweetheart deal using corporate funds. You're not allowed, even though it happens all the time, to use company funds for your own personal gain. You can pay yourself a salary, but that's it. Otherwise, in the hypothetical scenario where Microsoft comes after you, they could tip off the IRS and state tax collection agency that they may want to audit your tax filings, both personal and corporate.
There are legal ways around that.
For instance, a sole proprietor can setup a loan account where you are, in effect, providing yourself finance/goods under very favourable conditions.
I'm not arguing the point that it can be done, rather that after all the hoops you have to jump through to be fully compliant with applicable laws, how much time have you spent on it? Then how much do you value your time so that the $100 or whatever you might save still has you coming out ahead? If you value your time at say $25/hr, and it's going to take you 6-8 hours to save $100, you're losing money on the proposition. Granted this is a very low risk of anyone ever coming to enforce the terms of this contract, but they also don't really have to. MS could just nuke the key on the activation server, which costs them next to nothing, but you're stuck buying another copy of Windows. MS could just do that based on even the mere suspicion that you're up to no good and your only recourse is to take them to court and prove that they are in breach of contract. Even doing it pro se, the filing fees would probably be more than it would cost to go out and get a new copy of Windows. It's the same basic business model patent trolls use. You know it's a racket and that they intentionally set the prices just a little below what it would cost to even file a response to their lawsuit, but most people will pay it anyway because it's cheaper.
OK then, seems we're on the same page.