Spam is just the opposite, it is actually a pricing issue and not a service issue.
As long as the expected value per message of spam sent is positive, someone in the world will send as many messages as they are able to. You either fix this by raising their costs so that spam no longer has a positive expected value, remove their ability to send an unlimited number of messages, or both.
This is not exclusive to the e-mail system, robocalls are still a major annoyance, and the global telephone system is much more regulated. Mobile phones now automatically filter calls, even! It's wild.