It’s about safety.....FIRST.
Yep. There are at least a couple of very, very good reasons civilian aircraft don’t fly formations.
1) It’s dangerous. There is absolutely no reason to be that close to another aircraft increasing the odds accidental collision.
2) You start risking destabilizing the following aircraft due to wake turbulence. Sure, if you are talking about two large passenger aircraft the one in the back “probably” won’t get flipped over or anything but... things happen. You don’t go looking for ways to mess up the wing’s function and a stable flight envelope in the name of saving a few gallons of gas.
We take commercial air travel for granted on a massive scale. It’s as safe as it is exactly BECAUSE every little thing the aviation industry does for safety all add up to one very safe overall experience.
Start taking away a few here and a few there and soon the accident rate will go up.