http://php.net/manual/en/function.setcookie.php
grep -r setcookie in my code will let you find out if i sent any cookies. Incidentally, this is the same grep I would have to do on a framework, but the framework (pick one) is probably bigger than all of my code.
As an example -- we used to have a WordPress blog; now we have a custom blog that's a total of 3628 lines of php that lives on the frontend and I included the Makefile for deployment and some extra include files cause I don't want to spend the time to consider which ones aren't needed. Content is from text files pushed with another process.
By comparison Wordpress includes 298,643 lines of php. Wouldn't you rather dig through all of my code than all of theirs? (Yes, Wordpress has a bunch of features -- but even when you turn them off, the code is there lurking, and sometimes it still runs)
Someone else from my team fixed up the mobile support and handles the css/javascript -- they were able to just look at the code and do what needed to be done. (The mobile page was broken in our install of Wordpress too, so I got it to parity anyway)
It took me about a week to write it and polish it (three days dedicated, including exporting the content from mysql, two days I was doing other things too), but on the plus side, I don't need to spend half a day to figure out how to deploy a Wordpress upgrade every time they have a security release; there have been 12 security releases since then, so I've earned a day of my life back.
They wouldn't know vanilla SQL, HTML, WSGL, etc, --decade old standards-- etc, but they would know some random framework like Flask?
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?
At least that's why I would consider working on a team like that much worse.