I wonder whether the trend began as the amount of low quality click-bait on the web increased. 'Learn More' maintains a purposeful ambiguity - if users have read the summary, they're likely to have a fairly good idea of what they are reading about and whether they want to know more about it.

If there is not much more content to actually be seen in the article, it works in favour of those producing quick and easy content to hit metrics as they won't be over promising.