I hope there are suitable caveats around the Genesis text being mythology - I'd hate to imagine far-future generations scratching their heads at how otherwise technologically sophisticated people could image that this stuff was true.
lots of people believe it literally though and the majority of us are religous, we are exactly those otherwise technologically sophisticated people you are describing.
If you're a christian then creation/young earth is very important to protect, without it you have no "fall of man" and therefore no reason for a saviour. Accepting death before sin also complicates matters significantly (as you must if you accept evolution).
That and Jesus' legitimacy as a saviour is based off a bunch of prophecy and lineage from some of the earlier texts, including Genesis.
