Because if you insist that Genesis is literal for the creation on man, it appears massively hypocritical to not insist that Genesis is literal for the creation of the planet. Or is this a case where the science has been convincing enough to say that the earth wasn’t created in a few days?
And afterall, the account was writen for humans to read and understand. We use a mixture of the very literal and a variety of devices in our language quite confortably everyday with no problem, why should not another language do so also?