There are a few reasons for these crappy job listings. My wife works as a recruiter (not IT) but other branches of her company do.
" trouble and expense of getting recruiters"
This is not true. Recruiters bend every which way for clients. They only get paid when a person is hired.
"only to then dash out a half-baked job listing"
A lot of times, the people writing the job posting are the recruiters. By the time they get the requirements, the hiring manager has written something basic, then HR has played with it, then it finally gets to the recruiter who has to write up the final. By the time it gets on the site, they are so wonky because HR has messed with the wording That's where crap like having an internet connection, and comfortable chairs come from. The recruiter then adds tons of acronyms to cast a wide net for potential applicants.
A lot of the crappier postings are what are known in the industry as "Magnet Postings". Designed solely for the purpose of increasing their pool of candidates.