lets try another angle
So the only way to really "destroy" or "convert" ordinary matter is to pair it with the same amount of antimatter so that their "matterness" cancels out—but in practice there is almost no antimatter generally available in the universe (see baryon asymmetry and leptogenesis) with which to do so
As I explained a few posts back, the annihilation of heavy particles like protons creates a number of different particles, not just photons (what most people think of as 'pure energy'). But this process is not unique to matter-antimatter annihilations. Proton-proton collisions in the LHC result in the destruction of protons and the creation of large numbers of other particles too, as does any high-energy collision.