Works the same way as old-school junk mail. Your postal service gets paid well by junk mailers to put trash in your mailbox, so they're disincentivized to fix the systemic issue. I can't find a good quality source on this, but it's been said that about 45-50% of USPS & Canada Post's revenue comes from junk mail. They could fix it, but it would probably lead to a collapse of the entire post system due to revenue shortfalls. A true tragedy of the commons.