I don't intend to stand up and defend obviously terrible job postings, but consider the source here. The fact is that many people just aren't fantastic writers. Pragprog.com is a publishing company. If that posting were written by Dave Thomas himself, that means it was written by one of the most successful authors in the Ruby community.

Success in authorship means that you must be a good writer in general. Success in technical authorship means that you must be a good writer, as well as exceptionally good at organizing complex information (like a job description) and expressing it in terms that people can understand.

To answer the question directly: "Why don't they tend to look that?" Because most people can't write nearly as well as Dave Thomas.

Laziness, incompetence, and because the person writing a technical job posting often doesn't understand or value the skillset they are hiring for </generalization>