Email is ostensibly a decentralized system. But in its real-world use, it is centralized both at the application layer and at the protocol layer.
At the application layer, email clients (like the Gmail or iOS email app) are typically bundled with a reliance on centralized email hosting (running the mail server) and relying on their centrally-controlled name (@gmail.com, @icloud.com, etc.). This in turn gives the app developers significant leverage against the protocol layer, since they completely control how their users (through their app) interact with the protocol layer.
At the protocol layer, email relies heavily on a de facto reputation system, since any valuable and truly decentralized and permissionless system will have an abundance of spam. The reputation system is used to filter spam and isolate bad actors. This is why running a home email server is not really what this is about: it's about who you need to rely on to have your emails delivered.
The challenge isn't designing a protocol without reliance on any single actor. What's difficult is designing a protocol that can avoid becoming de facto centralized when critical features (like spam prevention, registration, etc.) need to be filled in by agents outside of the protocol.
The crypto people recognize this and that's why they want to build the economics explicitly into their protocols (a critically important part of any protocol; all protocols without exception have economic components whether one recognizes this or not). This is also why they emphasize global consensus and more.
Hacker News and others hate blockchains because they see the scams (which happen in all permissionless systems, and they are even more obvious when the financial components nakedly visible). Yet HN laments when the properties of decentralization that they love and value are lost for seemingly mysterious reasons ("dead internet theory", "enshittification"). The truth is that the reasons are right in front of us.