In some sectors it's to keep the employees in the same job from finding out the new recruit is going to be offered more than they are on.
My DH was not entirely happy with his lack of a pay rise and had asked me to have a quick look around the job sites to see if I could find a higher wage in his area of expertise.
I did find a higher paying opportunity. The advert was put up by a recruiting agency. The exact salary being offered was posted, duties that he seemed absolutely perfectly qualified for, same area as his current job so no change in travel arrangements...great I thought.
I was about to email his CV and application when I got this funny feeling.
Some of you have guessed where this is going I'm sure.
I googled some company description sentence fragments and found them in only 2 places on the entire internet...the job advert and DH's employers web site.
I'd only encouraged him to apply for his own job (more than one person doing identical jobs in his section) and his employers hold a very dim view if they find out an employee is looking for a new job.
On the upside, I did surreptitiously stir the shit a little and now DH gets the same as the new recruit who filled the roll.