Bitcoin wastes a lot of energy. I agree. It is basically a mechanism to turn energy into something analogous to money.
However, people do not generally live where there is a lot of potential to generate green energy. Wind, solar, geothermal, nuclear all generally work best far away from heavily populated areas (this is not always the case). Additionally, wind + solar are highly variable.
Per unit of energy renewable energy will always be more cost effective than fossil fuels in the long run, but there is a high fixed cost at present.
The more sources that can consume energy in remote areas the more incentives there are to build renewable energy sources.
Thus cryptocurrencies like bitcoin are a net positive for building green energy sources in the long term.
The energy used by mining Bitcoin is the energy required to hold back human corruption and fraud. In other words, it’s so costly because humanity is so shit. Fiat, by comparison, is cheap, but it is also inadequate at holding people back from corrupting their money.