Because the writing has been on the wall that people are no longer going to pay for operating systems. That means that MS has to replace all that Windows revenue with services revenue. Apple has already done this in that their services revenue now account for $100B annually (https://www.statista.com/chart/14629/apple-services-revenue/) and MS doesn't have a hardware business to rely on as Apple does.
I'm sure MS sees a future where Windows is free and if you don't have an account, they can't monetize you at all. At that point, you are no longer a customer so there isn't much point in them continuing to support you.
> Because the writing has been on the wall that people are no longer going to pay for operating systems.
I don't mean to be rude, but when has that writing not been on the wall? You can find scripts to activate Windows on fucking github. Like maybe people would be less apt to steal it if it wasn't so insanely simple to do that you could automate the process in a bat file?!
Add to it, I'd bet money the VAST majority of the time, people are using those things to install Windows to devices that already had Windows installed legally from the factory, with a legal license purchased by the OEM, that Microsoft refuses to, and has refused to permit users to reinstall with since I was a young bab in 1999, despite the fact that my GRANDMOTHER knows that the longer you run an install of Windows, the worse it performs, because it's poorly engineered.
Personally, I'd be fine paying for Windows if I could buy perpetual, permanent licenses for it that I could install with repeatedly, and that the OS wasn't constantly shoving ads in my face, haranguing me about using a stupid Microsoft account, and using enough telemetry to enable identity theft. But Windows licensing has always, always been exclusively the domain of PC manufacturers and corporations. Microsoft refuses to make it even slightly better to use for users, which is why users by and large just crack it and get on with their lives. $200 for a key you can use three times is fucking absurd.
~70% of desktop computing today is performed on MS devices (https://gs.statcounter.com/os-market-share/desktop/worldwide). Virtually all of that 70% is done on a paid operating system (mostly OEMs as you noted). While I think that it has been clear that we are moving to a free OS model since the Apple revival (iPhone driven, not desktop) the speed of this change is slow. There is no point in MS trying to rush it as they would just cannibalize what is still a ~$24B business for them.
How easy it is to steal something isn't really a good measure here. First, the vast majority of MS customers can not find a script on GitHub and run it and most don't have a desire too. MS is likely willing to live with some level of piracy as stamping it out would cause larger revenue loses than accepting it. That doesn't tell us anything about their strategy or the market.
As to the OEMs, MS is essentially doing what you suggest and offering a tiered model. If you buy Windows yourself at full price, then you can re-install it, change hardware, etc. If you get it through an OEM (where MS is selling that license at a massive discount) you don't have those rights. I'm not saying I love these choices - especially the BS about limited installs and hidden licenses - just that the choice does exist. (We can have a reasonable argument about the Windows tax - less important now, but still a thing).
If we're right about the future, MS has to replace that $24B somehow. Their current plan is services so they need to tie MS users to MS services and monetize those. In this case, it seems that MS doesn't believe that there are enough folks like you who are willing to pay > $100 for a license so they are pursuing this path.
Good news is - unlike the 1990's, we have multiple, credible alternatives so you can pick non-Windows systems.
They could at least learn from the tier model on planes. Pick the cheap and miserable experience where you'll get ignored the whole flight or the luxury seating with great attention and decent dining.
As it is now, Microsoft gives everyone below Enterprise the same bad ad-cluttered, zero-privacy experience. There's no motivation for an individual to spend more.
I am fine with that. If they provide any kind of valuable services I will make account with them and pay them money. So why not start with that small part of the project - make anything people want.
I'm sure MS sees a future where Windows is free and if you don't have an account, they can't monetize you at all. At that point, you are no longer a customer so there isn't much point in them continuing to support you.