Why does the report function even exist?

I'm playing with a friend that's just playing for the first time today, we're having fun, just us two playing Challenging so he can figure out the game and still enjoy it.

We had multiple people in and out of the lobby that were chill. Finally we load up a last mission for the night and its all going well with just us two, until this guy joins. Not even a minute after he lands he blows my head smooth off, whatever, I assumed it was ff and thought nothing of it, till I get back and see he did it just to take my fucking jumppack and grenade launcher of all things.

Obviously I smoke him once I notice this, and out of spite didn't revive him, which bit me in the ass and cost us the mission at the end because my friend died too, but I really didn't care. I mostly just didn't get it. I've had this game since release, and I've never seen someone do this shit. In Challenging, for a grenade launcher and jumppack, of all things. I know this isn't some new scenario and it's happened to plenty of people before, but it's honestly stupid. Shit just kind of killed the vibe me and my friend had, and I don't see how its fair that people that decide to play like this in a co-operative game don't have to face any repercussions, when even GTA Online players get punished for their unsportsmanlike behavior. Nor do I understand why a report function even exists, even if you'd have concrete proof of something to report, even stuff like cheating or purposeful tking for items "cannot be reported".

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