The technology exists, but time and time again, we hear that real-life terrorists used unencrypted communication. They weren't caught because nobody was looking. Criminals and terrorists are not law-abiding, but more to the point, most of them are not awfully bright.
If I were advising the security services, I'd be looking to put out messages calculated to encourage villains into using particular means of communication, where anything they might leak would be less needle-inna-haystack than the sum of all 'net traffic. One way to encourage that might be to have politicians and officials call for particular apps to be banned, thereby sending out the message that diabolical plots can be safely shared using precisely those apps. If any such app happened to have a backdoor, the calls to ban it (or force it to introduce a backdoor) would be loud and clear.