EAGLE has always had a HUGE base of user supplied libraries, that is probably one of the reasons for its early success.
But, of course, many of those are not up to standard, not even those from "established" sources like Sparkfun, Adafruit and similar.
You REALLY cannot blame Autodesk for the errors in those and ignorance of the creators.
However, I agree that it would be great with a"verified by Autodesk" label, that guaranteed that the library is correct and follows certain standards.
Without that, there is absolutely no time saving in using pre-made libraries, as you would need to go through all the pin assignments to check that they're correct, check the footprint to see that all pads are where they are supposed to be, as well as of the right dimensions.
I can make a complete component in a fraction of the time it would take to check somebody else's work.
My library contains 1000+ parts that I've made over the years, it's rare that I need to make a new footprint at all. If I do, it's usually very specific, and not one you would find in a "standard" library.
Also, the library system is so f*cked up, it's almost unusable. Slow and confusing. Another reason for not using it.