Why does my phone battery die so fast?

They die because batteries are essentially a type of electrochemical cell. An electrochemical cells works by reacting 2 different metal solid electrodes with a solution containing the same metals but in an aqueous solution. For example when magnesium electrode is put in a beaker containing magnesium nitrate solution and a silver electrode is put in a silver nitrate solution and the two electrodes are connected together we get electricity flowing through the wire. However when this occurs electrons flow from magnesium electrode to the silver one. When that happens there is a build of a negative charge on the magnesium electrode. At the same time there is a build up of a positive charge on the silver electrode. So we connect a tube called a salt bridge to allow some of the ions in the silver solution to travel to the magnesium solution and vice versa. Now that you know how a battery works you can understand how it stops working. Now as the exchange is happening through the salt bridge to balance the charges on both sides there is a build up on the electrodes. This build prevents the electrode to come in contact with the solutions they are submerged in. When that happens the reaction stops and the battery dies.