If crimes were easily trackable and prosecutable, we probably wouldn't have such a hard time keeping criminals in jail. We still have crimes committed in jail that don't get properly solved, much less sentenced.

Also, this isn't necessarily about dumb kids being busted for drugs or stealing, but organized crime members. The possibility of them ordering hits from inside prison walls or tracking down people in witness protection kind of makes people nervous about letting them talk with anyone and everyone. That's not to say it's the most effective means, since smuggling, messengers, visitors and other means still exist for passing information. But that's not really an excuse to not try at all.