It is not because government cannot do it, it is because it is LOW Priority.

When a rat is found in a bottle of wine, about 15 years ago, the response from our fellow (nice)customer is: some many manufactures, so many productions, it is impossible to fix the problem. Actually the fix is very easy: replace that rat with some anti-government slogans, or reveal some "state secretes", see if every bottle will be checked!
Another easy solution is: poison one Gao Gan or his relative, see if you get high alert.

Button line: who cares you fellow grass roots, as long as you don't gather together and turn the ruling party upside down, your voice won't be heard.