Because God said He would.... And I believe him.....
Logic would say that man has messed up God's words while it was being copied and translated.... Human logic usually fails to see God's hand (Greek seek after knowledge).
Rom 1:20 KJV
(20) For the invisible things of him from the creation of the world are clearly seen, being understood by the things that are made, even his eternal power and Godhead; so that they are without excuse:
........Logic keeps most man from believing.
Heb 11:1-2 KJV
(1) Now faith is the substance of things hoped for, the evidence of things not seen.
(2) For by it the elders obtained a good report.
It's the invisible things we are to have faith in...
Psa 12:6-7 KJV
(6) The words of the LORD are pure words: as silver tried in a furnace of earth, purified seven times.
(7) Thou shalt keep them, O LORD, thou shalt preserve them from this generation for ever.
...It may not immediately LOOK like God did what He said He would do, but His hand has been preserving His words for over 4000 years. I have faith that He is.