Say you've build a weather app/website. Being a good citizen, you publish "weatherevent" objects. Now anybody can consume this feed, remix it, aggregate, run some AI on it, new visualizations, whichever. A great thing for the world.
That's not how the world works. Your app is now obsolete. Anybody, typically somebody with more resources than you, will simply take that data and out-compete you, in ways fair on unfair (gaming ranking). You may conclude that this is good at the macro level, but surely the app owner disagrees on the micro level.
Say you're one of those foodies, writing recipes online with the typical irrelevant life story attached. The reason they do this is to gain relevance in Google (which is easily misled by lots of fluffy text), which creates traffic, which monetizes the ads.
Asking these foodies instead to write semantic recipe objects destroys the entire model. Somebody will build an app to scrape the recipes and that seals the fate of the foodie. No monetization therefore they'll stop producing the data.
In commercial settings, the idea that data has zero value and is therefore to be freely and openly shared is incredibly naive. You can't expect any entity to actively work against their own self-interest, even less so when it's existential.
As the author describes, even in the academic world, supposedly free of commercial pressure, there's no incentive or even an anti-incentive. People rather publish lots of papers. Doing things properly means less papers, so punishment.
Like I said, incentives. The incentive for contributing to the semantic web is far below zero.