What makes it work is the asymmetry. With normal human communication you talk to the same person thousands of times, or send thousands of messages from the same

What makes it work is the asymmetry. With normal human communication you talk to the same person thousands of times, or send thousands of messages from the same account, so spending computation equivalent to something like $1 worth of Bitcoin for that is totally reasonable. Spending the equivalent of $1 worth of Bitcoin to send one spam to one person, isn't. Even if you had access to a large number of computers, you'd make much more money by using the same computational resources to mine cryptocurrency. The opportunity cost makes spamming uneconomical, and if it doesn't then you raise the cost until it is. You can also allow users to pre-approve each other to avoid paying anything so that most legitimate communication doesn't require proof of work, which makes it a lot easier to put a high price on the remaining instances.