Well, Flask is 95% just two persons [1] and 5% among 10 others. And the one responsible for creating it and writing 60% of it doesn't even have it as his first priority (has multiple other libs, codes Go, etc).
A lot of widely used Javascript libs are not even that lucky. Heck, GTK+ itself, one of the pillars of Gnome and used by millions, was down to 1 maintainer a few years ago, who openly complained about it (not sure if things improved since).
Unless it's some large, focused project with bit traction, tons of books and courses on it, etc., like React, Angular etc (usually with some corporate backing too), a lot of the things that pass for serious frameworks and libraries are anything but, and a lot is hardly a "community" project either.
Some person slapped together something and thousands adopted it because "frameworks are better than writing your own code". Then the person moves on, or some small team that leads the project decides to rewrite it from scratch to try new stuff because they have short attention spans, and you're left with an abandoned code in your hands, which you haven't wrote yourself, and don't have time to understand fully.
A framework can be good to start with if you're doing stuff close to its confines, but otherwise its how Joe Armstrong described OOP: "You wanted a banana but what you got was a gorilla holding the banana and the entire jungle".
Notice how all major companies, from Facebook and Google, to Twitter and Microsoft and Apple etc have written their own frameworks.
Perhaps their programmers didn't know that already existing frameworks are better? Or were they focused on their actual problems and pain points, instead of lowest common denominator solutions?
They wouldn't know vanilla SQL, HTML, WSGL, etc, --decade old standards-- etc, but they would know some random framework like Flask?