Especially since once they complete it, then they have to weed through the initial wave of inbox abuse people will be reporting.
I can't find it anymore but im pretty sure it has already been said that the messaging system just isn't as important as a lot of other stuff on the site so it will take a while
@Volvagia has said in the Discord that it's not being worked on it right now, as @blueYOSHI said it's just not that important for the amount of time and work it would take to create it, since forums, Discord, etc all serve the same purpose as messages would and do the job adequately.
The messages icon will probably get removed from the top bar at some point so that people stop asking about it.
I once tried to make a blog software from scratch. Sounded easy, just a title, the text, maybe the date, repeat until all posts are there, right? Well I did work on it for a week and never actually finished it and what I have now is basically just parsing a json file and already bloated up to multiple days of tinkering. A message system needs much more work, and maybe even encryption, unless you want site admins to be able to read your messages to other users. Then there'd also have to be some form of spam filtering, which judging by how often I see bots on this forum, is not a thing that's present yet.
There is basic spam stuff I guess from what I seen on the discord but it will take a while to make messages everything will need to get a whole load more complicated, plus its such a small thing for that much work. I guess if you want to contact a mod on site use forums and if you want to private message them you can message them off site :)