Personal walkie-talkies work on a smaller range of frequencies and they send out analog signals. These are basically "raw" signals without any special encryption or compression or processing. An analogy would be with light waves, and shining a flashlight towards your neighbor's window. Anyone with eyes who knows which house is going to be sending a message can look at that window and see the light.

Cellphones are on a larger range of frequencies and they send out digital signals. When you talk into the phone, it gets processed into bits (0s and 1s), sort of like recording a sound file. And that takes up space, so the phone then compresses it so it takes up less space (and also reduces the quality of the audio), and then sends that over several varying frequencies so it can all be received at the same time at the destination location, where it then has to be decompressed and converted back into an analog signal (sound). And there are different ways of delivering that signal to the target phone, for instance, it might get cut up over a few different frequencies or sent out along with other calls on the same frequency (just received and decoded by the appropriate recipient). All that means that even if you happen to tune in to the same frequency that someone else's phone is broadcasting at, you're not necessarily keyed in to the same timing and signal that it's sending at, and because it's digital your phone won't actively be decoding whatever it is receiving because it's not on an actual call.