Not just that, but the tooling has also evolved and changed.
Good luck trying to get an old project with old versions of Yarn and Gulp working well, where trying to update either breaks the build but using anything other than var confuses the tooling. Provided that you have a build step of any sort (e.g. minifying the assets) and people picked whatever was popular at the time of the project being made.
It's moved on massively as a language, picked up many, many new paradigms.
Going back to code more than a few years old and your newer devs are going to be asking "what's all this var nonsense"?!