Impure churches. That's the wheat mixed in with the tares. I wonder, are there holy churches? For real, not just claiming to be.
The odds of getting a bad apple would seem to dictate that if they exist, they are tiny (dozens at most). And it would be virtually impossible to tell.
The tares do often serve some kind of constructive function in the churches; God doesn't just have them there to frustrate the wheat. The proof is that he will not uproot them now "lest he hurt" the wheat.