Yeah, I can't wrap my head around why they would do that.
Is it really a big enough revenue stream to inconvenience the consumer to such a degree ?
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Maybe it is because the type of user that uses wordpad is also not concerned with advertisements or user experiences.
I mean, for most people a computer is either a Netflix + Internet + Office machine (they live in these apps, so the rest of the OS doesn't really affect them) or a device which runs significantly worse OEM software,(in which case, good UX was a lost cause anyways.)