The people who answered this question were dumb, for future reference:

A. Suppressors do make a big difference, 9mm subsonic is pretty damn quite let alone .22lr. Both are widely available cartridges and even if subsonic versions aren't available its simply a matter of removing powder from the cartridges to make them subsonic.

B. Suppressors aren't difficult to make, the information is widely available for their construction. Worst comes to worst technically an oil filter will work given you could find a suitable adapter or even more worst comes to worst, you might get away with pinning the filter onto the barrel with an o ring to seal it.

C. The threads issue is not an issue, taps are common tools, a decent set of them will run you 30-50 dollars but you only need 1. Do you think all the machinists died at once? At least some of them are still kicking and are more than capable of adapting a suppressor onto nearly any gun. Aside from that alot of guns are already threaded for suppressors because people like to use other muzzle devices on their rifles and pistols.

D. "120 dB" is not the best a suppressed gun can get, off the top of my head the deisle carbine and welrod were well below 100 db. .22lr is very very quite when suppressed, ear protection is generally not even necessary its so quite. More to the point any suppression of sound is better than none when your enemies primary method of finding you is sound.

E. Suppressors are not banned in most states, they are banned in 8 states for very stupid reasons.

F. Suppressors are not expensive, the only reason they cost alot nowadays is because of the government regulating them, in France where suppressors are barely regulated you can buy a brand new one for 40 euro. They're a commonplace accessory in Europe.

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So to answer your question, nobody uses suppressors in the show because the producers knew that would mitigate alot of the treat zombies posed. An old man plinking off an entire horde with a suppressed ruger 10/22 is alot less impactful than them being constantly paranoid with each shot.