You must not have used Flak Jacket in Black Ops. In Hardcore, it allowed you to survive most explosions other than the RC:XD if you had not lost any health. This was because they added a separate damage reduction modifier for HC modes. IW did not implement this in MW3 until a few months ago, and the damage reduction percentage they set is still not high enough to counter the splash damage from the majority of explosives.
The problem isn't that explosives can kill you, it is that a person can roll with up to 11 explosives, every single spawn, and doesn't have to be remotely accurate to kill you. Spam, spam, spam, and if any of them hits a wall or the ground even 20m behind you, you're still going to die whether or not you use the Flak Jacket perk.
If the perk wasn't in the game, we'd just have to deal with it (and continue complaining) but the fact the perk is there, and you sacrifice the benefits of Assassin/Quickdraw/Hardline/Overkill to use it in exchange for no benefit is the problem.
