The problem with CSS is that if you want to write an SVG viewer, you have to implement a whole CSS engine, which might be more complex than SVG renderer itself. And if you create an image in an editor, like Inkscape, you don't use CSS anyway. CSS is meant to be used when you write the code manually (instead of using an editor), for example, in a web app, and in this case you could use HTML as well.

So yes, CSS is not needed.