How is a transistor power calculated
The power rating is based on the thermal design of the TO-3 transistor package, how easy or hard it is to extract heat from the transistor die inside.  The MJE3055 is basically the same die in a smaller TO-220 package, and is rated for only 75 W.  The thermal path from the die to the mounting surface is better than the old TO-3 package, but that mounting surface is significantly smaller.