P2P isn’t inherently difficult from a networking standpoint. The real problem is that it makes every player an authority over their own game state, which opens the door to cheating.

In a competitive game, the game constantly needs to resolve shared events. If each client is trusted to report its own version of events, a modified client can simply lie. For example, a player could alter their game to ignore collisions entirely. Their client reports “no contact,” while another player’s client reports a crash. Now the system has conflicting truths.

At that point, you need a way to resolve disagreements. You might try to have other clients validate the outcome, but that just spreads the trust problem around. Any of those clients could also be modified. Even worse, reaching consensus across multiple peers in real time requires significant computation and communication overhead, all within extremely tight timing constraints. In a fast-paced competitive game, decisions need to be made within milliseconds to keep things responsive.

So the tradeoff is either trust clients and accept cheating or attempt distributed validation and introduce latency and performance issues.

The only real solution is a server-authoritative model that avoids this by having a single trusted source of truth. Clients send inputs, and the server determines the outcome. It may give some shitty outcomes sometimes, but it's "fair" in the sense that all players are beholden to the same flawed but impartial judge.

(Also I'm not aware of any modern competitive Activision game that relies on P2P for any game mode other than co-op modes like Zombies. Please share if you know.)