No, it is not impossible to discover backdoors and vulnerabilities in closed-source Windows and Mac software; there's a cottage industry of third parties doing that on a pro-bono basis.
But can we really trust companies that work with the NSA to sell us the version compiled _without_ the backdoors?
Plus, always keep in mind that these companies have lied to us about PRISM - with straight faces.
That being said, it is probably feasible to hide a backdoor even in open source software. But it is still easier in closed source software.