Why would it be necessary for a muscle cell to have a higher percentage of mitochondria than other cells in the body?

It is necessary because through kreb's cycle and ETC, the mitochondrion produces a LOT of atp per glucose compared to glycolysis alone, so the atps are then used as energy for the muscles

The mitochondria is a energy machine for cells and since your muscles are always moving, they have to use a lot of energy. Without the higher percentage of mitochondria, then you would not have enough energy to move at the drop of a hat