People mine bitcoin because there are millions of dollars per day of bitcoin rewards available.

I would guess that collectively, millions of dollars are being spent each day on cancer research around the world.

These mining farms were built for a purpose. It’s not like all these spare ASICs were laying around waiting for somebody to find a use for them. The people running these farms would be on some other venture if cryptocurrency was never invented. And those ventures probably wouldn’t involve curing cancer either.

Greed is usually the answer. Cancer research doesn't pay. Bitcoin does. If someone could just harness one to do the other. If there was a crypto currency whose calculations helped to cure cancer then we'd be on to something

People as a group are selfish and short sighted, prone to maximizing personal gain. Individuals can be self sacrificing and possess a long view, but there will be a thousand others willing to work the system to death in their place. More specifically, there are almost limitless rationales to be employed if you stand to gain from turning entropy into personal wealth. “This compute power was never going to be used to cure cancer anyway...” that kind of thing.

More pernicious are people who cloak themselves in ideology or Utopianism somthst they convince themselves that this waste of resources is a good thing. Some may even believe it, but most just don’t care and are too busy finding greater fools so they can make money.