Just as you cannot solve the problem of harmful emissions or pollution by measuring individual harm and somehow tracing that back to the source.

How would a person trace their illness caused by PFOA emitted by DuPont, if DuPont could simply counter with "prove that the specific PFOA molecules that made you sick came from our plant."?

It doesn't work that way, and for damn good reason. You don't have to prove specific individual harm, in order to condemn and regulate actions that do harm on societal, national or global scale.

Cryptocurrency demonstrably does harm by vastly increasing global power consumption, plus the cost and environmental damage from producing the increasing amounts of hardware needed to run crypto as a whole.

That is why regulation is needed, because we cannot let the misguided desires of a few crypto-obsessed tech heads contribute to climate change on such a scale.

why not

It's not obvious at all. A title of ownership that doesn't grant ownership of anything at all is, in principle, worthless.