I think the average solo entrepreneur or small business owner doesn't have the time or skills to learn and practice
SEO. It's a lot of work. And even if you can afford to hire it out, you have to know enough to choose a competent service.
The other factor that I rarely see discussed is that Google is a for profit corporation. There's nothing wrong with that, but it has to influence everything they do. I'd be willing to bet that their bottom line trumps whether or not a website "deserves it" when an update wipes out their search traffic.
(Any by the way, I think that if you haven't found a website that didn't "deserve it" you haven't looked very far.)
So back to the original question, "So why do we see people promoting having your own hosting and site and doing
SEO..." Maybe it's because they still believe that if only they'd do it right it would work. Plus, in general people are resistant to change. It's comfortable to keep doing what we're doing.