Hi Cary,
because P/B Frames only store a part of the Key-Frame. In a Long GoP it simply takes too long between the Key-Frames. Again, I refer to animated Photos or Images, not about Movies which are a total different thing. BTW, "no GoP" would mean Key-Frame only (I-Frame). I think you are familiar how compression works
I suggest you do a Test yourself with static Images or Photos in a Long GoP to see for yourself, then you might understand what I mean okay ?
Cheers
Mick