Why is such a complex molecule such as ATP used as a "currency" for energy

Our body makes energy by breaking a phosphate group off ATP. If energy is made from electrons jumping to a different energy state, why not use simpler molecules such as CO2 as the "currency" of energy?

Because of the complexity of ATP the molecule is stable at different energy states.
Oxygen is an excellent electron holder, it is highly electronegative, and there are four of them bonded to each phosphate. So the cell is free to make and break bonds of ADP/ATP without completely destroying the molecule, or worse creating a highly reactive molecule (:CO from CO2) or a free radical that would go on to destroy more important molecules in the cell.
Not to mention the "relative" ease of bond making/breaking with ATP because it is so stable, to try and break a CO2 bond would be difficult and require probably more energy than could be obtained from it.

Well ATP is the main source of energy, CO2 is what we need to breath out in exchange for oxygen. It is often called the "molecular unit of currency" of intracellular energy transfer.[1] ATP transports chemical energy within cells for metabolism. It is produced by photophosphorylation and cellular respiration and used by enzymes and structural proteins in many cellular processes, including biosynthetic reactions, motility, and cell division.[2] One molecule of ATP contains three phosphate groups, and it is produced by ATP synthase from inorganic phosphate and adenosine diphosphate (ADP) or adenosine monophosphate (AMP).