The reason the idea is not implemented is because it can easily lead to abuse. I know you're imagining a system where someone no longer benefits while they are AFK, but the mere ability to take yourself out of the action at will, is a benefit. As as been stated already, what stops someone from using this whenever they find themselves under attack or surrounded by enemies? They can wait for the enemies to pass or be dealt with by someone else, then pop back into the action.
To solve this, someone could suggest some specific rules for using the AFK option. What if you can't use the AFK option if you are within some minimum distance from a zombie? Perhaps you can't use it if you've been hit by a zombie, or fired a weapon in the last 5 seconds. For any rule we come up with, someone's going to run into a real life situation pulling them away from the game, and they won't be able to use the AFK option because of that rule.
In Swamp the AFK type of option was already put in place, but with a specific set of rules for how it can be done in order to avoid abuse. In Swamp, you can exit out of an ongoing mission any time you want, as long as you walk into the truck and press Enter. This prevents you from using it suddenly to escape a death, and because doing this removes you from the mission it prevents you from using it to just avoid danger and then return to normal once it has passed.
I'm asking about other games and other developers to show the conclusion that nearly every development team reached, which is that you can't just let people pause the action whenever they wish. If you play something like World of Warcraft, you can teleport yourself back to base if someone knocks on your door. To prevent abuse you can only use it if you're not actively attacking or being attacked, you have to stand still for about 10 seconds without being attacked as the spell casts, and when you get back you have to walk however far to get back to where you originally were. It lets you deal with real life, but has restrictions and consequences for it's use.
League of Legends has a similar system for teleporting back to base. Most games have nothing at all. If I'm playing MineCraft and my cat starts throwing up on the sofa... I am completely at the mercy of the game's enemies and other players until I return.