A: Among other reasons, because Jesus and so many others in the Bible authenticated the Old Testament and referred to Genesis as scripture. See Matthew 19:4 and Mark 10:4-9 for two examples where Jesus quotes from the Book of Genesis as scripture.
Jibu (J): Pamoja na sababu nyingine, kwa sababu Yesu na watu wengine wengi kwenye Biblia walithibitisha ukweli wa Agano la Kale na walikielezea Kitabu cha Mwanzo kuwa ni Maandiko (Matakatifu).