Because PoW is designed to use as much electricity as possible. If electricity becomes cheaper (as a consequence of providing more electricity), PoW miners will simply use more of it.

Because we afraid of nuclear energy. There are lots of fossil fuel fans masquerading as green activists who claims that nuclear is kind of no eco-friendly. And yes, the possibility of building great things is decreasing nowadays since the best minds are working on letting us to see more ads.

Why? Because right now everything we do creates more CO2.

Producing more energy means more CO2.

Even producing more solar panels creates more CO2.

The problem is CO2 and climate change. your and mine climate change.

It will cost us less to avoid producing the mass amount of energy for Bitcoin than trying to make it real CO2 neutral which is very very far away