Because they want you to use a MS account. One that can track everything you do for *cough* "advertising" *cough* purposes.
This is the same company that thinks saying mean words is worthy of banning you from ALL games you have ever purchased from them on your account. The same company that keeps trying to tie everything into a cloud subscription service they control.
Money and control. That's all this is about.
You can complain about control, (even mac os still allow local accounts) but money? Microsoft wouldn't last long if they weren't interested in money.
With their new business model, we lost some, but we also won some. I'm pretty happy that the windows 8 licence that Igot as a student is the only purchase that I had to do, rather than spending hundreds once a windows version is eol.
You can still buy a lifetime licence to office, but they are losing ground to G-suite and its cloud/collaborative-based approach, the last company that I worked in used G-suite and gmail, so MS have to adapt. A lot of product/brands have gone extinct when they couldn't get with the times.
Google having access to so much user data/ads allows them to propose a lot of services for "free", everything is transactional, and in the current times, data/ads is probably cheaper than real money as far as the user is concerned.