True enough, the regular refs were human too and sometimes botched calls or missed them. The difference here is the number of times it happens (in particular) and what types of mistakes are being made. If the replacement refs were doing as good a job as the regulars guys, meaning they blow a call every now and then, we wouldn't be hearing anything like what we're hearing now.
Also, the issue isn't just the missed calls, but the absolute uncertainty on the part of these officials. The games that used to take 3 hours are now taking 4 hours, and its all because the officials take too long to decide what they're going to call. I have seen numerous instances of the officials needing to go to replay before even making an official call on the field... which is a major no-no.