The trouble is that this is both boring (for a person) and hard (for a computer.)
So nobody wants to do it manually, and while everybody's got an algorithm to mark up text, they're all terrible and prone to being gamed by unscrupulous advertisers.
How many websites have you gone to and seen some random word in the middle of the text that's bolded, double-underlined, larger font and a completely different color to really draw your eye to it (and away from what you're actually there to read.. ie: be as annoying as fucking possible) and then you hover over it and discover its a Wikipedia link to a house or something equally as pointless?
This has been the problem with "the semantic X" ever since link farms were invented. They usually don't provide a whole lot of additional information (if any) and they distract from what you're trying to see.
If you really want a semantic experience, go to basically any popular wiki. They're explicitly curated and therefore the links you find are (usually) actually both informative and relevant. Of course they do this by going the boring (manual) route and compensating for it by having a million people doing the job instead of just a handful.
Go back and read that "mundane" Wikipedia article about the house and, if you have even the slightest amount of curiosity about anything, can probably spend several hours link chaining.. there's links to construction, history, archaeology, anthropology, etc -- and they're all placed in such a way that they're relevant to the article and yet kept subtle enough that you can read over the ones you aren't interested in without a significant drain on attention.