How and why did the top journals become top journals? Journals like Nature and Science have impressive impact factors. How and why did these top journals become top journals? Why are they able to sustain their statuses?
All journals that have a high standing have the standing because of the support of the community. If the community loses interest, the journal will drop in the ranking. The top journals have therefore attracted authors for one reason or another. The editorial staff of journals try to maintain this status by making sure the work published there is of good quality and will be cited. It is thus not impossible for new journals to attain high status as long as authors provide the necessary papers. To this mix, we now also add the impact factor and other bibliometric factors. They matter now but have not been the driving factor for making the older journal what they are today.

I work with a more modest journal and I can definitely state that improving your impact factor is far more difficult than to drop in ranking. But, if your ranking becomes high enough (no specific number will be relevant since it varies between fields) a journal will be self-fuelling since many want to publish their material there and competition stiffens leading to a strong selection.

So the standing of Science and Nature, is part their long history, in part the hard work by the journal itself and in part the, now, need for authors to publish in as high ranking journals as possible since that is what forms the basis for most evaluations in academia.