So, in Goblet of Fire, why didn't Barty Crouch Junior just turn a 랜덤 cup 또는 book 또는 something into a portkey and trick Harry into touching it?

I voted Shelly best answer, because that IS the answer in fact.
Nonetheless there was an explanation to this. They said, I'd guess, Moody (aka. Barty Jr) said that they had to go through the whole Triwizard Tournament so that Harry's planned death would seem an accidental tragedy of the Tournament and Voldemort's return would not be uncovered. That's why it was so important for Barty that people won't spot him behind Harry's success.
Then in the Order of the Phoenix some Order member (Lupin maybe) says that Harry screwed up the plan for Voldemort when he stayed alive and told everyone about his return. This was the scene when Sirius got the Order to answer some of Harry's 질문 concerning Voldemort.
So basically the whole thing was organized in order make Harry's supposed death seem to be an accident and thus keep Voldemort's return secret.

becaus then it would be a really short book

So it would seem like an "accident"