Why haven’t those who may be terrorist, insurgents, etc. not discovered how to use onion routers or similar technology to communicate in the blind?

If technology exist that can bounce your communications through a myriad of computers while encrypting the date many times as to hide the source and designation, how come those seen as insurgents or terrorist to the governments they oppose have not learned to use it? I am sure some criminal organizations have learned to use it to their advantage.