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Nothing is scarier that the unknown and is this trope is based on that fact. Nothing beats the human imagination in its capacity to terrorize, well, itself, or the owner of said imagination. Showing the monster too early ruins the effect. Your mind has something to focus on, something to process. With that information in hand it can calibrate the fear factor to a manageable level. But absent the requisite information, there is nothing for the human mind to process, but process it must. Thus the expectation of what the monster is fills the imagination terrorizing the audience far more than the actual object of terror.
Timing is the key to this trope. It is not a question of if the monster will be revealed, it is monster delayed not denied after all. Often the reveal is timed with the story climax for maximum tension. The focus shifts from the monster to the victim and whether or not they can escape the horrible fate store for them. Maintaining that tension is a tale for another trope.