Why do cells tend to have more ADP molecules than ATP molecules

The reason why the cells have more ADP than ATP molecules is that the cells have the tendency to use more ATP, rather than restore ADP molecules.

When the body produces energy, in order to be able for it to be released in the body and to be used by the body, the ATP molecules break up. During that break up of the ATP molecule it actually loses one phosphate group, thus becoming an ADP molecule. Since this process is going on constantly, the ATP molecules are constantly breaking up thus resulting in constant new ADP molecules, so they are easily becoming outnumbered.