Much of the quran was not written down during Muhammad’s lifetime. It was transmitted orally by the “reciters” (hafiz) who supposedly committed it to memory as it was told to them. Even the stuff that was written down by his scribes was changed; the scribe Sarh renounced Islam after recognizing that Muhammad let him write down Allah’s revelations however he pleased.
It was only after he died that people started consistently writing it down and compiling it into codices, and even then there were as many variants as there were reciters.
And it was already known that many parts had been lost.
https://quranx.com/Hadith/IbnMajah/DarusSalam/Volume-3/Book-9/Hadith-1944
It was narrated that 'Aishah said:
“The Verse of stoning and of breastfeeding an adult ten times was revealed1, and the paper was with me under my pillow. When the Messenger of Allah died, we were preoccupied with his death, and a tame sheep came in and ate it.” 1: These verses were abrogated in recitation but not ruling. Other ahadith establish the number for fosterage to be 5.
Allah’s “perfect word” was already unreliable.
It was Uthman, the Third Caliph (third successor to Muhammad), who edited and rewrote it, with the intent that his quran replace all the other qurans.
https://quranx.com/hadith/Bukhari/USC-MSA/Volume-6/Book-61/Hadith-510/
Narrated Anas bin Malik:
Hudhaifa bin Al-Yaman came to `Uthman at the time when the people of Sham and the people of Iraq were Waging war to conquer Arminya and Adharbijan. Hudhaifa was afraid of their (the people of Sham and Iraq) differences in the recitation of the Qur'an, so he said to `Uthman, "O chief of the Believers! Save this nation before they differ about the Book (Qur'an) as Jews and the Christians did before." So `Uthman sent a message to Hafsa saying, "Send us the manuscripts of the Qur'an so that we may compile the Qur'anic materials in perfect copies and return the manuscripts to you." Hafsa sent it to `Uthman. `Uthman then ordered Zaid bin Thabit, `Abdullah bin AzZubair, Sa`id bin Al-As and `AbdurRahman bin Harith bin Hisham to rewrite the manuscripts in perfect copies. `Uthman said to the three Quraishi men, "In case you disagree with Zaid bin Thabit on any point in the Qur'an, then write it in the dialect of Quraish, the Qur'an was revealed in their tongue." They did so, and when they had written many copies, `Uthman returned the original manuscripts to Hafsa. `Uthman sent to every Muslim province one copy of what they had copied, and ordered that all the other Qur'anic materials, whether written in fragmentary manuscripts or whole copies, be burnt.
Except it’s clear they weren’t.
On top of this, Arabic did not have standardized rules for consonants or vowels until much later, making it a poor choice for the creator of the entire universe to convey his important message.
And then the cherry on the top is that the more recently discovered, older manuscripts such as the Sana’a manuscript (DAM 01-27.1). It’s a palimpsest, which means it was reused, and has an original text (lower text) underneath the upper text, revealed by ultraviolet and other analysis. These older manuscripts have significant differences from the standard readings, such as not mentioning Jahannam (”hell”) at all.
In many cases, a sentence in the older version stops, where the (relatively) more recent version embellishes. For example, an older manuscript might say that Allah will “punish the disbelievers in this world”... and then stop, while a more recent one (forgery) will add “and the next,” making it a whole bigger ballgame.
As one scholar (Gerd R. Puin) remarked:
https://www.theatlantic.com/magazine/archive/1999/01/what-is-the-koran/304024/
"The Koran claims for itself that it is 'mubeen,' or 'clear,'" he says. "But if you look at it, you will notice that every fifth sentence or so simply doesn't make sense. Many Muslims—and Orientalists—will tell you otherwise, of course, but the fact is that a fifth of the Koranic text is just incomprehensible. This is what has caused the traditional anxiety regarding translation. If the Koran is not comprehensible—if it can't even be understood in Arabic—then it's not translatable. People fear that. And since the Koran claims repeatedly to be clear but obviously is not—as even speakers of Arabic will tell you—there is a contradiction. Something else must be going on."
The claim that the quran is the direct and unchanged words of Allah requires one to ask the follow-up question: exactly which quran are you talking about?