Well, there may have been a period when MS was trying to improve product quality, and in that time, yes maybe they did have very comprehensive automated testing processes. But before that, up to the time of Windows 7 I guess, their quality was dog shit.

In the early days, MS was an undisputed monopoly though, and not only did they not test thoroughly, they hardly even tried to fix bugs - the userbase had to take care of that too. Earlier versions of Windows had all sorts of workarounds and 3rd party tools to try and get things to work properly.

I suspect that once they'd achieved their objective of improving quality, there just weren't the incentives there any more for middle management to allocate resources to things like comprehensive tests.