Undergroud objectives, like Jackleg Joe- All alone?

When you are in a mission that goes underground, why can't go with a person you've been grouped up with, doing EVERYTHING above ground? I've seen a couple instances of this. It especially irritated me to go after Jackleg Joe, alone. All Single player like. I was definitely having a good time sharing the workload with a buddy, when topside.

So is the transition to a dungeon based objective just programmed wrong? Whereas, a couple of peeps should be able to stick together thru thin 'n thin? Sharing vehicles, multiple objectives, and Arkfalls shouldn't be any different than questing underground.

Do many boss fights end up like this later on? What about this Nim? -Thanx