This is not a community site in the sense you are describing it. Those still exist.
Community messaging is not the goal of itch. It is not built around it. To slap it on as an afterthought can be done for small groups where half an admin can deal with anything coming up in their spare time.
It would be different if there was a separate community forum here and you would need to make an extra account. But as it is now, the same account is used for everything. Private messages would be available for anything. To write to developers, to commentators, to random people.
I seen some ugly discussions in public comments on games. If those were be able to be held in private messaging, plus if you could PM developers, that would be that moderation nightmare, that was cited above.
Plus the spamming. They do not have enough staff to sort actual scam malware projects out. What makes you think they would have the staff to handle private messaging? The public forum here is mostly (visibly) handled by one moderator. And that moderator is not staff. Because it is a small forum, the public forum that is.
As I said, the situation might be different, if we would have to make separate accounts for public forum.