Because Microsoft sees the 1/10 of a penny it gets from the ad as more important than your Desktop and your Taskbar.
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Edit: I am dumb. Please disregard.
edit: <snark now unnecessary>
And that they’d rather is $0.001 over your desktop experience as your desktop usage doesn’t generate them income.
Early sources, such as this classic book
https://www.amazon.com/Image-Guide-Pseudo-Events-America/dp/...
frame public relations and marketing folk as cynical manipulators who don't fall for the illusions they create.
The post-modern environment has had about 60 years to develop since that book was written and I think the culture has deteriorated and I believe that, today, the image makers really do live in the "the matrix" they've created and have trouble distinguishing it from reality. (I think of how Valmont in Dangerous Liaisons really fell for his victim Madame de Tourvel.)
My contacts with local business people and people who sell ads for radio and newspapers have convinced me that prices for advertising were too high for a long time because the people who buy advertising get gratification from hearing their name on the radio and the people who advertise won an auction because they get more gratification from hearing their name on the radio more than anyone else.
Years ago somebody who was an "influencer" would try to appear authentic (not an "influencer") but today authentic people will try to look inauthentic ("i pretend to be an influencer even though I don't get paid") to get credibility.
People under this spell will run ads in situations when they clearly shouldn't and they'll be astonishingly resistant to anyone who tries to talk sense into them.
(Your desktop usage does make income for Microsoft since your OEM bought Windows for you. If they annoy you enough you might switch to Linux, MacOS, etc. Unfortunately your OEM probably wants to wreck the value of a $2000 computer they sold you by selling $2 worth of annoying ads.)