Could you use this structure instead: 现在世界上最高的人是谁？
That is accepted.
why is this not an acceptable answer? 现在世界上谁是最高的人
世界上最高的人 "tallest person in the world" is considered a unit. Your sentence sounds like "now, in the world, who is the tallest person?"
Colloquially, I think it's acceptable, but the given one is better.
What is the purpose of the 上？ Is the question correct without it?