Obviously you can encode S-expressions in XML (including symbols). But you have to add additional structure to do it. The point is that XML, following the markup metaphor, doesn't work this way out of the box. And, in fact, I've never seen anyone (except, I guess, you) go to the trouble of making all the distinctions in XML, such as the string/number distinction, that S-expressions make -- and I have seen people get into trouble for failure to do this.

It's a psychological/sociological point rather than a technical one, but metaphors matter in design.