Because people tend to put way too much dang oak in their oaked beers.
This is where I think the factor of time/exposure comes in. And you need to taste the beer as it ages to determine when you've hit the level of oak you want.
I've had commercial beers that were just awful from the oak. La Trappe oak aged quadruple jumps to mind. The regular quadruple is great, the oaked stuff I never need to taste again.
this is true but it's important to remember that AFTER it gets too oaky it gets better. I find that at first, in the first week to 10 days if can be very overwhelming and harsh, but then around week 5 or 6 the character of the oak changes and softens. More complexity develops.
I had this impression the first couple times I used oak and then At that NHC presentation he confirmed my theory to some extent. As the beer penetrates the oak which takes a long time even with spirals (though not at long) it picks up very different characters.
I do remember him mentioning either as part of the presentation or during the questions afterwards some guidelines for how much to use. I do think surface area is what you want to look at rather than cubic inches think square inches of contact surface. (as narvin said) It also varies based on the kind of wood in question.