Someone asked a question along these lines on Facebook recently, asking what one piece of evidence in particular persuades people to adopt the view that they do.
There are multiple things that I find particularly indicative. The reference to a dome in Genesis 1 is itself significant. But the point becomes even clearer if one knows other creation stories from the Ancient Near East.
In the Enuma Elish, the Babylonian creation epic, Marduk slays Tiamat (their version of the more familiar Hydra, a seven-headed dragon representing the sea). He uses half to make the sea and half to make the sky, fixing an arch to hold up the latter. In other words, waters above and below, the former held up by a dome or arch or whatever one wishes to call it. Just like in Genesis.
The author of Genesis clearly did not have a different view of the natural world than the Babylonians who wrote about creation before he did. He had a different theological view, envisaging one sovereign and powerful Creator. The way the author expressed himself would have been understood to reflect a view of the sky and of the wider cosmos that he shared with others in his time. To insist that the author of Genesis used language that directly parallels what others in the Ancient Near East wrote, but meant something that only much later readers would understand, is to treat Genesis 1 with the utmost disrespect, not to mention dishonestly.
If we were talking about the account in Genesis 2-3, then I'd point instead to the presence of a talking snake. Unless one starts by imposing on the text particular presuppositions not required in the text itself, one will recognize quickly that the presence of a talking animal is an important clue provided by the author, indicating that the story is not a literally factual one. We know how we are supposed to understand stories with talking animals, and it is only indoctrination persuading someone to set aside everything we know as readers that can result in such a text being read in the radically inappropriate and forced manner that many fundamentalists read it. Insisting that the story has to be historical-factual rather than symbolic, when it is not introduced with such a disclaimer about its genre, and the clues within the text would naturally lead a reader to conclude otherwise, is once again to treat Genesis with disrespect and dishonesty.
The evidence seems pretty clear. So why do so many people insist that things are otherwise?