It's been the default since Windows 95, and I suspect it's because it was just noise to the average and upcoming computer user (ignoring the fact that coolsong.mp3.exe would soon become a problem, since that probably hadn't happened yet). MacOS functionally did the same, so MS probably wanted to seem equally 'sleek'.
The fact that it continued to be the default past Windows 98/ME/2000, when problems caused by it were widespread, baffles me though.
I can't find a source to find an exact reason though, and given it's been a thing since 95, there might not been one out there.
It might have varied across versions (maybe Home Edition still hid them, while Professional didn't?). But given the number of guides and people complaining about it online even in the XP days, it must have been a default in some capacity on XP.
It was still a default on some Windows 10 installations I've made, so it's still around (can't confirm 11, since I haven't installed that for myself).
So true! Does anyone know why this happened?