Thats because POW solves the Byzantine Generals problem as I understand it. Before POW, that problem was intractable (extremeley non-trivial). Its always lammented that so much energy is needed to solve the problem, although that seems to be the nature of the problem. Maybe time and energy are inexorably linked.
Well it's a bit more complex than that but the two are certainly linked given that POW is framed in the context of time complexity of the problem and the adjusting difficulty is just an equation to approximate a clock from the rate of solutions produced for a problem of a given time complexity with an approximate X amount of available resources.
