> How is email 'effectively' centralized?

By having big provider turn up the spam detector for small actors. It's currently common to have a perfectly set up mail server, with verification and reputation and longevity… and still have Gmail flags the emails that come from it as spam.

Also, forget about sending email from home. Email sent from residential IPs are instantly deleted, and not even sent to the spam folder, by big providers (Hotmail makes it an explicit policy, and bounce the email right back). You have to at least relay the damn mail through a non-residential IP.