In the world outside of schools, one rarely - if ever - goes to a meeting without having any real idea about what it's for, what will be the focus of discussion, what the intended outcomes are, and whether any prep is needed. There is usually some kind of pre-amble; some one-way or two-way communication in the run up to the event, which sets both the scene and the expectations.
Why not for parent's evenings too?
Actually, I know the answer. It's because an email to a parent from a teacher about their child will almost certainly get a response, and there is so little time available for meaningful interaction, that just the possibility of this is enough to make it not happen!
But one can control communication. One can set parameters. And one can assign the task of pre-communication to an administrative role, especially where it follows a formula. And one can switch roles, so that it is the parent who does most of the thinking and writing in relation to their one or two kids, rather than the teacher having to do it in relation to their 30, 60, 90, etc.
I would love it if my kid's school sent me an email two weeks ahead of parent's evening, asking me to submit details of what concerns me, what delights me, what I'd like to understand better, and what I'd find it most useful to discuss.
If an administrator had collated these responses and presented them to the teacher so that s/he had 3 minutes to review them before meeting me, then I am happy as I have my own agenda, s/he's happy because s/he's met my requirements, and the aims have been fulfilled.