In addition to cost concerns (which yes, do apply to AAA devs, they don't get carte blanche): Theoretically P2P can lead to lower latency if the players are clustered geographically. If I'm playing with my friends in Phoenix, it's a lot faster to send packets directly to their machines than to send them to e.g. Colorado and back. The advantage is even more apparent when you're dealing with, say, Australia, who historically get -blam!-ed in their server choices. This assumes, of course, that the host has enough upload to send packets to everyone, which isn't set in stone. You have to then build your game around this, which limits how much can be actually going on. Even worse, Bungie went with a full mesh network for PvP, i.e. every player sends packets to every player. It sounds like a terrible idea. Frankly, I'd think that they're idiots, but it takes more work to do that than to keep what they had in Destiny 1, so there must be some benefit.

I’ll tell you why it’s cheaper