Probably two reasons. One, they want to keep the option of shuffling the rank available. Reasons for shuffling could be because someone receives a new award or publication that bumps them up, someone gets an IA and that moves them down, or like
@gyngyn said their class becomes unbalanced (possibly in terms of M/F ratio or race/ethnicity, but also traditional vs nontrad, research focuses applicants vs service focused, etc). The other reason is they don’t want to set false expectations. For example if someone is told they are in the “top half” of the waitlist but they don’t get in, you’re likely to get an angry email later about what happened, how far down the rank list they went, etc.
Ultimately, even if you had the information, it isn’t actionable. If you aren’t holding an A then you need to be proceeding as if you need a gap year. Even if that wastes a little money if/when you manifest the A, it is going to set you up much better in the long run.