Why can't humans hold their breath for a long time?

When we breathe, we inspire oxygen and we expire cardon dioxide. The oxygen and carbon dioxide are transferred in tiny blood vessels surrounding the alveoli. This prossess occurs through diffusion. However, as humans, we need to inhale and exhale very frequently. There are several reasons for this. These reasons include the fact that all our organs need oxygen to function. Another reason is because cardon dioxide has a higher level of acidity than oxygen (measured with the PH scale). When the acidic carbon dioxide remains in our bloodstream for too long, it reacts with the blood and sends signals to the brain to let out the carbon dioxide and replenish the oxygen.