But then you are just duplicating the file extensions in the directory layout. DRY, and all that. Some people are anti-file-extension, but I'm actually not in URLs (and definitely not on disk) at least insofar as the extension indicates the content type of the response, not the content type on disk.
There's so many ways to cut it, maybe "static" is the best because it's just the least complex.
Like everything else in software engineering, this depends on your needs.
Have one js file that everything needs? One simple .css file that everything uses?
Sure, stick them in static.
Have a bazillion php files, each depending on a complex system of rules to generate the various CSS files it's using? Things get more complicated.