Sites like Hexbear source scripts from only one domain (hexbear.net). I like those sites.
Other sites loop in a few JavaScript libraries. They have a few domains asking for permissions. That's okay. A little lazy, but acceptable.
Then you have the corporate websites with, like, sixty JavaScript domains. Holy crap! What are scripts from facebook, pinterest, and amazon doing on this news article about jellyfish? 
And the website is perfectly functional with just a couple domains enabled. 
I think it was out of a desire to reduce staff tbh. Around the time the concept of a full stack dev was becoming popular and the idea is one person does everything rather than splitting the various components into different specialties and since js was mandatory on the browser why not get the frontend devs to write the backend as well. It was a tech bro psyop thats my theory.
What sort of incompetent no-long-term-planning 3-second-attention-span halfwit would ever think that using a dynamically-typed scripting language with barely any standard library and which is famous for undefined behaviour would make a good backend language, just to save a few dollars in the short term, was ever a good idea?
Oh, right, just every tech company upper-management drone on the planet.