It costs millions of dollars annually to store and transmit spam; I've seen estimates that up to 80% of email volume is spam. It's also the #1 malware vector resulting in billions of dollars of damage, cleanup, and lost productivity a year. "Just pass everything through to people's spam filters" really isn't practical, even though the countermeasures are a fucking pain in the ass to deal with.

Email is a fundamentally broken protocol, basically, and 30 years of bolting fixes on top of it hasn't done anything but make it a fundamentally broken protocol with lots of annoying things on top.