The scale of the plagiarism is something that's come to light after one instance finally got tagged. It was a starting point that lead everyone to all this. What starting point would IGN have had? The only opportunity would have been after his hiring IF an overseeing editor or manager somehow had watched or read whatever article he plucked from AND noticed some similarities. The only real opportunity you could say was the borrowing from his coworker. In that case the similarities might have been noticed because there was a chance that the same overseeing editor or manager looked at both. But what if two different people checked those reviews/previews? Again, unless they had some reason to suspect Miucin, why would anyone think there was anything amiss about his article? There's an argument to be made here that internally, articles/scripts could be fed through something like what they use in academia, but this is not the norm for the vast majority of companies. You just don't suspect your coworkers, your professional peers of malfeasance unless you have some kinds of signs.
In terms of the vetting process, again, it's not the norm to treat potential hires like the CIA would. Criminal checks, references, degrees, heck, even credit checks, but you're not going in there with the expectation/suspicion that their portfolio is anything but their own original body of work. Again, you could make an argument here that they could invest in some kind of anti-plagiarism tool, but unless they've developed one for podcasts, youtube transcripts, etc., I don't know how useful it'd be. Basically, you're asking for a level of scrutiny associated with jobs where lives are at stake, not entertainment gigs. Is that realistic to have expected it of IGN? Is it even realistic to expect it following this?