Why publish a research paper when a blog post or a lecture slide can have more citation count than a journal paper?
It's not fair to only look at the peak of the distribution. For an apples-to-apples comparison, you need to compare peaks to peaks and averages to averages.
The two sources you mention are both in the field of machine learning. If we assume that they correspond to the blog/lecture notes sources with the most number of citations (i.e. the peaks), then we can conclude that these venues can generate at most ~700 citations. If you compare to the most cited machine learning papers, these 700 citations are minute. For example putting &quot;machine learning&quot; into Google Scholar yields:

Scikit-learn: Machine learning in Python (journal article) -- 14919 citations
Data Mining: Practical machine learning tools and techniques (book) -- 34724 citations

What about averages? I don't know what the average number of citations a blog post or lecture slide gets, but I'd guess less than one, since many blog posts don't attract comments. The average number of citations for a journal article however is easy to find - just look at the impact factor. Putting in &quot;machine learning journal&quot; into Bing, I get journals such as Machine Learning (IF = 1.855 as of time of writing) and International Journal of Neural Systems (IF = 4.58). Clearly the average journal article gets a lot more citations than the average blog post or lecture slide.
tl; dr: What's the point of studying machine learning instead of playing soccer and earning hundreds of thousands of dollars a week? The answer to that question is similar to the answer to this one.