Because it’s believed to be “literal word of god” revealed to Muhammad by an angel word-by-word. Since Allah is perfect and all knowing his word is considered to be flawless and eternal and not open to interpretation or doubt. The theological argument is that imperfect human reasoning will lead to misguidance and thus corrupt the religion so only literal interpretation is acceptable.

So it’s like a constitution that cannot be amended but it’s from over 2 millennia ago when most people didn’t even know how to read or write. It’s a dead end.