True, matter is an energy... however the only known way to convert matter into pure energy is to make it annihilate with antimatter ( akin to electron-positron). We first have to have a good and cheap source of antimatter! And the matter annihilation may still generate a lot of particles/matter - that may be a waste (or not) depending what you do with that further.
A Partial conversion of a matter into energy is also fusion and fission reactions, but they are limited by the difference between source and end element of reaction here https://en.wikipedia.org/wiki/Iron_peak#/media/File:Binding_... - e.g. H2 -> He3, That uses only a tiny portion of matter, no energy that can be extracted from Fe56 (max), and all its close elements are also useless for either fusion or fission energy extraction.