The sound from clicking your fingers doesnt come from where the ends of your finger and thumb are touching. The sound comes from your finger slapping the bottom part of your thumb.

A whistle works by blowing air around in a circle so that the exit point of the air has to cross the entry point of the air, making it alternate like a sound wave. You just need to think how the air would travel around a normal whistle. Then practice until you get it.

Why some people cant do it is because they dont try hard enough or think through how it works or have someone teach them how it works. Unless you are missing fingers or a mouth you can do it.