Christians believe that only one thing is necessary to be believed: that Jesus died on the cross for one's sins, to give his imputed righteousness to those who believe (justification by faith). Any who say or imply that it is necessary to believe that physical creation took place in any particular way (or anything else) are, they say, either antichrist or ignorant.
Early Genesis is nonsense if it is taken as a textbook of physical creation, because its two separate 'accounts' of creation must contradict each other in various ways (though some translations, notably the NIV, have tried to disguise these contradictions). The stories are undoubtedly allegories carrying much spiritual meaning, which can be discovered fully by reference to the Hebrew texts. That they are allegorical is obvious even to children reading in their native tongues.
Those who propose a young earth, Young-Earth Creationists, YECs, have four motives. They firstly wish to shift the basis of Christian belief onto an intellectual plane rather than one of a change of will. This results in phantom conversions, with people thinking they are Christians when they are not (and there are plenty of those already in the USA). Second, YECs aim to direct attention away from the spiritual themes present in Genesis chapters 1-11, which, even today, many real Christians are unaware of, to at least some extent.
Third, YECs aim to give as much importance as they can to Sundays, which is the day on which they encourage brief association of as little as an hour per week as a token commitment to Christ. The concept of a creation period of six literal days assists this practice. There is no dispute here that the Israelites did not keep a seven-day week; but that week was itself a prefigurement for Christianity, which itself is a 'sabbath' or rest from attempts to work for salvation. Works salvation is the principle of many sorts of false Christianity, so even those who are not themselves literalists may favor the spread of the young-earth view.
Fourth, by insisting on a creation account that is so much in contradiction with biology, geology, archaeology and even history, YECs attempt to ridicule Christianity and bring it into contempt.
