People were already building substantial web apps long before the current generation of popular libraries and frameworks were around. Some of those applications are in use to this day, jQuery and all, and still in active development. They can include a huge amount of functionality after that much work has gone into them.

Would a team choose the same tools if they were starting those apps with a blank canvas today? Probably not. Is anyone going to throw away a decade of working code to rewrite the apps that already exist? Also probably not.