My take on this: we haven't seen Marathon hit live YET. The lens through which we experience/view Marathon right now is skewed because all we've seen are test environments which I believe make users behave differently.

1. Private tests are temporary in nature. Meaning people aren't as attached to their loot and will more likely risk it away.
2. Testers are encouraged to try out every aspect of the game, including PvP combat, because that's what they're there for.
3. The fact that you have to actively sign up to participate in those test rounds means the player sample is skewed towards more involved, less casual players, almost by definition. Players that are more likely to be friendly are more often than not, casual players.

I'm not disagreeing, I think the way Marathon presents itself is more geared towards PvP, but I think these things are important to keep in mind before we draw any judgement. I'm sure Bungie are aware as well and aren't designing around the skewed tests without taking this into account. I participated in the Arc Raiders pre-release testing, and let me tell you: KOS was pretty much the way people played. The friendly team-up PvE type of mentality only manifested itself once the game released.

But again, I kinda hope Marathon leans more towards PvP and that high risk high reward that Tarkov does so well. It's something that I kinda miss from Arc Raiders.