why does steam causes more severe burns than boiling water
It is not that the steam is hotter than the water. Water's boiling point is 100°C, so it is vaporized upon reaching 100°. Therefore, the steam is 100° also -- no more hot than the water.
The reason that steam inflicts more damage has to do with what happens when the gas changes back to a liquid. As the steam comes into contact with your skin, the molecules slow back down and become a liquid. The states of matter are defined by the speed of particles (based on the amount of kinetic energy within particles) and how "stuck" together they are due to intermolecular forces. Particles that are separated and are moving fast will be a gas, slower moving particles will be a liquid, and the slowest being solid.
So the molecules are in a gaseous form and are moving really fast. The reason they are moving fast is because energy (in the form of heat) has been constantly added to the water molecules until they got enough energy to break apart from the others. When the vapor comes into contact with your skin, the vapor condensates and changes to a liquid. What this means is: the molecules are going from high movement (and energy) to lower movement (and energy) as a liquid. The energy (heat) is being lost in the transition from gas to liquid. Heat is being released as a result of the state change. In an equation: gas → liquid + heat. The change is an exothermic reaction; heat is given off.
You add this heat from the change of gas to a liquid plus the original 100°, and you will get a worse burn. A burn from water would only be 100°, but the burn from steam is 100° in addition to the exothermic reaction.