1) Because the pay is low and they want to either i) show the employer they can bring possible candidates (if recruiter) or ii) try and talk you in via other benefits.

2) They dont want to publicise the pay to other staff especially if staff are under market rate.

3) They have a big range and they dont want to show their hand if they bring someone in more aligned to the bottom end.

Generally I think its number one and wasting peoples time not saying.