Because while it may be realistic, powerful explosives are awful for gameplay balance. People just end up spamming them like in COD4 and MW2.
Also, more underpowered but quirky perks like in COD5 (Shades, Reconnaissance, Gas Mask, Flak Jacket, Toss Back, Fireproof). I had Shades on some of my classes just so my guy had the cool sunglasses. The Flare resistance was worthless. But I mean they might as well put them in. As long as they aren't overpowered, it wouldn't hurt. I was disappointed at how few perks MW2 had.