Those kinds of systems tend to have been created and popularized before any single large actor appeared and started using it. Push notifications were single-actor from the very beginning. Decentralized setups in that situation are much more expensive to build and run - with the exception of "from the goodness of their hearts" projects, a business has little reason to make the sole implementation vastly more complex and open to abuse.

And tbh I'm not sure SMTP serves as all that positive of an example. It works, sure, but the reputation system and extreme complexity that has sprung up around it makes it difficult for newcomers to join - I can't just host one from my house, most email hosts will reject my emails. It works in spite of all this because it has too many large actors already using it for any new ones to stand any chance of rejecting (or improving) it.