They needed to, in order to make the proof-of-work cheap to verify. See my blog post http://cryptorials.io/beyond-hashcash-proof-work-theres-mining-hashing htt

They needed to, in order to make the proof-of-work cheap to verify. See my blog post http://cryptorials.io/beyond-hashcash-proof-work-theres-mining-hashing http://cryptorials.io/beyond-hashcash-proof-work-theres-mini... on how asymmetric PoW avoid this handicap.