Because the bonds connecting molecules of a substance need to be broken in order for something to boil. These bonds can't just break on their own, instead this requires some form of energy.
"For every action is an opposite and equal reaction" This well known statement should be a clue to why, as the boiling of a liquid requires a stimulus for the molecules to become excited enough to become steam, which itself gives of energy as heat. (Very basic but you should get what I mean)
Because the boiling of water cannot take place without some form of energy. Something needs to do the work to change the state of a liquid.
Heat is itself a form of energy. You need to use energy for heat production. There has to be something which makes the molecule to move faster and quicker. For this purpose, energy is required to break free from each other and to exceed the atmospheric pressure pushing down on the liquid.