Why don't publication venues systematically make the LaTeX source of papers available?
I suspect that this is a solution in search of a problem. Most likely journals aren't making LaTeX versions of papers available because they don't see a demand for such a service from the side of the readers; this would take effort to implement and maintain; and some authors would object to the idea of making it easier for others to reuse (read: plagiarize) their papers. In other words, publishers simply have (or at least think they have) better things to do with their time and money.

It should be noted that arXiv does make LaTeX source code available, and in fact will only accept submissions in the original source code rather than as PDFs (for papers that were written in LaTeX), so in those areas of math and physics where uploading preprint versions of one's paper to arXiv is the norm, this "problem" (such that it is) is already solved.