How is the RPG 7 still blowing up all our modern combat vehicles?

Somebody please explain that to me, how is a 60 year old weapon STILL a threat to the most advanced 21st century combat vehicles? that's absolutely ridiculous. The damn thing was invented in 1961 and yet in 2011 it can still blow up our supposed advanced vehicles. It's not even a smart weapon, let alone a SABOT or HEAT round, it's just a goddamn grenade with a rocket on the back of it. Rocket - Propelled - Grenade.