If it's easy to get history people will assume everyone is reading what they're typing. This would make people feel obligated to play catch-up every time they miss some messages due to being AFK or in the zone.
By making history hard to obtain, IRC simulates real-world conversation -- you're either in the room or you're not.
I'm beginning to feel this way about everything. Reddit, Twitter, social media in general...
The hoarder mentality likes for this stuff to still exist in the hopes it might one day be useful. In fact, it has very little value.
Snapchat got it right when they started with ephemerality.
Chats in organizations are specifically for when email is not realtime enough but it's important to keep a record of the conversations for some time.
This varies immensely per organisation so it's always kind of dangerous to generalise like this. For example: The use-case you quote does not align with my experience of (bigcorp) chats at all.