Sucker Punch clarified: matchmaking for the raid will not appear on release. It will become available "in a couple of weeks." Players are encouraged to find a group through the official Discord.
On the one hand, this makes sense – the developers want teams to be formed from players who are willing to communicate and act in a coordinated manner. The chance of finding such players is higher than in random matchmaking.
But in Ghost of Tsushima: Legends, there was matchmaking for raids, and after studying the mechanics, even random groups coped without serious problems.
A technical reason is also possible – the system needs more time. One way or another, for the best loot and cosmetics, you will either have to look for a team or wait for an update.