Which is even more a factor when you realize that neither book was ever intended to be written as a book. The Quran was a recitation. It was designed to be spoken, not written. Arabic wasnt even a written language at the time.
They did start writing them down. But during the reign of the "Rightly" guided Caliphs, there were varient texts going around and they ordered the book compiled and the varients destroyed. This is historical fact.
There is more than enough time for humans to have corrupted the Quran before it was even compiled to a single volume.
The Bible wasn't meant to be a single volume either. The revelations were written down and then debated and discussed about their accuracy who the revelations were to and whether they were accurate. etc. Some people mistakenly believe they are in chronilogical order when they aren't. And while I dont doubt that God guided the process, I am not of the opinion that He forced people to keep the book free from error. Especially when it never claims to be free from error. (seems to me that if the Bible was to remain free from error, it would be a pretty big error to leave that doctrine out).
The fact that human imperfections may touch the scriptures doesnt take away from the Divine message. As 2 Timothy 3:16 states, the scriptures are given as a tool to bring us to God. They dont need to be exalted in place of God. I fear there are alot of people who make their Bible an idol and miss the invitation to come to Jesus Christ and know Him and the Father.
It's another reason I like the Book of Mormon. Because it doesn't claim perfection. It points out that this is written by men inspired by the Lord who are not perfect and not incorrupt. But instead, go to the Lord and find out from Him whether the doctrines are true.
I know, if I wrote something inspired of God, I would want people to learn from God about it rather than simply relying on what I said. its better to trust the Spirit than the Flesh. Even Flesh as trustworthy as me