The facts in Quran is one type of its miraculous nature, other types are telling the real stories from history and future that Mohammad (PBUH) could never know them by himself, also telling about scientific details that were not known at that time, but the one which is related to your question is the linguistic challenge.
Quran was revealed at the golden age of the Arabic language, Arabs mastered the language to a great level, they did not need to study it, it was main part of their culture, they were able to say the poetry on the spot due to the level of fluency they reached, at that time when all people mastered the language, the challenge came to make it clear that the style and the build of Quran chapters or verses cannot be by a human, they had all the time and people but no one was able to provide a similar text.
As it is clear to language experts how linguistically miraculous Quran is, it is not that clear to people who do not speak Arabic or not so fluent as the original Arabs, but by reading on the subject you can better capture it.