My peanut butter of choice consists of nothing but peanuts and salt. Why doesn’t it taste exactly like salty peanuts?
Spanish peanuts also get used for PB.
Plus, a very large part of what we think of as “taste” is actually texture or mouthfeel, and that’s certainly different between whole peanuts and peanut butter.
And on top of that, there are tasty compounds found in peanuts that are not released by simple crushing. You have to really grind them fine.
In peanut butter the peanut oil is “right there”, even floats to the top of the good kind. But you don’t generally don’t get an oily mess if you chop peanuts with a knife or crush them with your teeth. So actually grinding them so fine the oil is released both substantially changes the mouth feel and texture, as Chronos mentions, and adds more readily accessible peanut oil taste.
And why doesn’t hazelnut flavored coffee taste like hazelnuts?
Because of the coffee. 
Since I love eating natural PB straight from the jar, panache’s question had been in the back of my mind as well.
The Dope, where your questions can be answered even before you ask them! 
But, wait, that makes it sound like the same peanut: the Virginia peanut. Or at least when I think of peanuts, I think of what you eat at the ballpark. That said, I’ve never noticed a big difference between ballpark peanuts and Planters. They all taste like peanuts to me.
Because it tastes like filberts.
There really ought to be a comic strip or something with an old married couple named “Hazel” and “Filbert”.
It does, which is why I never drink it!