Why isn't the block hash used to calculate accumulated work?
The answer is because among the set of acceptable hashes, every hash is equally likely. See https://bitcoin.stackexchange.com/a/939/60443

There really hasn't been more work done just because the value is greater, as long as it meets the current difficulty. Nickel and diming the work done by comparing block hashes would create many more forks, thus requiring more confirmations to be sure a transaction is final.