Have you ever SEO'd a site? That is something that certainly takes semantics into account and does so in a way that affects every page and their relationships.
Interface design is a separate process entirely, and its focus should not always be constrained by the engineering it would take to actually code the interface.
If it would take so little effort to make such a software, it would certainly exist. The mockup tools out there are pretty primitive and also fairly limited in number. What you describe is certainly in the future, but to be truly good it will require some very sophisticated technology.