Proprietary software typically does everything within its power to stop you introspecting it.

Also, Windows is just suspicious in general. It's slow, everything makes network requests. Finding malware in Windows is a needle in a haystack. For some perspectives, Its all malware.

It is difficult to find out why Windows is slow again. My colleagues using Windows complain about it regularly, but not not even one ever started an investigation whether there might be backdoor or not, because this would be hopeless. With open-source it is feasible.