Never understood that either. The cutscene is there for a reason. I've probably had more experiences with gameplay portions I wish I could skip than cutscenes. I can remember a couple instances where I felt the cutscene went on too long, but generally they are well used and provide some variety, flesh out the characters/story, etc..
Personally, I've always thought the distinction between "gameplay" and "cutscenes" was a little over-simplified. Gameplay is not just about twiddling sticks and pushing buttons, as Intelly points out in the chess analogy above. "Gameplay" also includes thinking. For example, thinking about how the game is currently set up -- where the pieces are on the board, how the pieces interact, what might be possible strategies, what might be the consequences if I do X or Y, etc..
Cutscenes, meanwhile, are described as passive activities the player "just sits and watches." But is that true? Do you cease to think during a cutscene? In adventure games or RPGs, a cutscene will often serve to arrange "how the game is currently set up -- where the pieces are on the board, how the pieces interact, what might be possible strategies, what might be the consequences if I do X or Y, etc.."
Granted, not all cutscenes are like this -- many of them do play out as passive viewing activities and just serve to take the story from point A to point B. But I've seen plenty of cutscenes (typically in RPGs, adventure, or action/adventure games) that have made me think about how the "game board" is set up, how the "pieces" interact, what my strategies might be, rethink my next moves, etc. So, cutscenes aren't necessarily just passive viewing. Sometimes they involve active thought about alignments, moves, choices/consequences, etc. within the "gameplay" itself.