If energy is conserved, why do people get tired, and why are we running short on "energy supplies?"

Because energy is being consumed. It takes energy to lift things, walk around, etc. We are transferring our chemical energy from meals to our environment and basic functions. The answer to the second part is probably that so much energy has been transferred and stored in our surroundings, and it's difficult to gather that energy into a useful medium for us to use again.