Except it does not actually increase the cost of scraping meaningfully. Compute is really cheap. The compute for minting an Anubis cookie will cost less than a thousandth of a cent even assuming the attacker uses the same JS implementation of proof of work rather than an optimized native implementation. That cookie can then be used for hundreds of requests.

How big of a deterrent is a millionth of a cent per page going to be? A non-existent one. Just the bandwidth from a residential proxy will cost the scraper orders of magnitude more.

Proof of work just isn't a viable counter-abuse challenge, even for something as low-yield as scraping. (It might be economically viable as a counter against some types of DDoS attacks, since they're even lower yield. But in practice it just moves the attack surface to the proof of work validation service.)