What if your content doesn't have enough views / you get bored / life changes so can't afford server costs anymore / die. Your server will expire eventually, and there goes your content. web.archive.org might have some sites archived, but many blogs won't have been archived so their content is just gone forever.
I've self-hosted many platforms, and many have died, perhaps due to running costs or lack of need anymore. It's partially why I'd never self-host my email, for example.
You can't know the future, and if your content is gone, it's gone for all of your audience (or potential future audience). Perhaps an ideal solution is some kind of self-hosted platform that mirrors content into a forever-public external repository, and hence preserves it, even if your hosting ceases to exist.
I also have a bunch of old blogs that I've just given up on at some point and now they're gone forever.
Your last point is kind of what archive.org's Wayback Machine does. Haven't checked whether they have an API where you can submit URL's you publish* or if you'd have to submit new posts manually, but this could be a decent solution. It wouldn't cover non-tech people though.
*) I just now quickly browsed through their API doc but couldn't find anything clear, other than uploading random files
With self hosting I can control the code, what it does, the cost, where it stores data, how backups are made, etc.
It’s always a trade off on control, and my comfort point is “on AWS”.
For some people they want their own servers. Some people want to own the network block. It’s just what level of comfort you have with each type of control.