I think SEO is a large reason why popular sites do bother with semantic tagging.
For example, somebody had build an app that scrapes free recipes from the web, so that it's shown without ads and the ridiculous fluffy lectures that are typically part of it.
Great for users of that app, not so great for the recipe authors depending on this model.
This is also why RSS is dead. Both Twitter and Facebook had it before, but killed it. You don't want to give away your goods for free.
It's easy to put this problem at the recipe bloggers or Facebook, but we're just as responsible for this. We don't want to pay for anything so it's ads. Then we avoid and block ads.
We lack a payment culture.
Then there'd be no reason to optimize the web to serve up ads, because legally you couldn't.
The world needs to switch away from advertising for the good of humanity.
* A significant number of people do not really use file names. https://jayfax.neocities.org/mediocrity/gnome-has-no-thumbna...
* Twitter offers no official way to make bold or italic text, but some people make it work using math symbols. They either don't know, or don't care, about all the accessibility and search tooling this breaks.
* Try downloading a C program off the internet and compiling it on anything other than the original developer's computer.
In none of these cases does anybody benefit from the lack of usable metadata, yet nonetheless the metadata doesn't exist.