Why? Why is this still a thing for co-op focused games in 2022?
Redfall

Redfall is an upcoming vampire slaying game that is still in development at Arkane Austin. In the few times that it has been shown off, an emphasis has been placed on the cooperative experience. It was initially shown off with a trailer that was very much like, "this is a co-op game through and through!"

Just this past week, we were finally shown some gameplay of Redfall that, you guessed it, emphasized the co-op experience. Arkane even showed off how players will be able to combine their powers and abilities with their teammates to create some pretty devasting combos.

So why then, with all this emphasis on the co-op experience, has Arkane made the decision to tie campaign progress to just the host of a co-op game?

Game designer Harvey Smith, in an interview with IGN, says that campaign progress will only progress for the host of a co-op game. This means that if you join somebody else's game and complete a level, only the host will get credit for that completion. You will still need to complete that same level in your own game to progress.

Smith says that the team at Arkane had originally intended for all players in a session to get credit for a level completion. However, he says that it would have been too confusing for players.

"So for the flow of things you want to have to redo those. The story would be very confusing if you got to mission 8 and it said 'skip this one because you've already done it.'"
Would it not be just as confusing for players to jump into a co-op game that is on mission 8 when they haven't even completed mission 3 in their game?

Why not just give the players the option of replaying missions they completed in someone else's game? Give co-op players credit and if they get to a mission in their own game that they completed in a co-op game, just have the game go, "Hey, you already completed this mission in . Would you like to replay it?"

Smith at least says that character progress, including any loot and XP you earned in a co-op session, will carry over from another person's campaign to your own.

It was also revealed that you will not be able to switch your character once you've started a playthrough. You will also be unable to respec your character, meaning skill tree upgrades are permanent.