I blame the declining popularity of world globes. Now that people can only see the Earth on flat screens or pieces of paper, of course they're going to think the Earth is flat too.
If gravity is real, how do planes stay in the air?
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It is easy to go down into Hell;
Night and day, the gates of dark Death stand wide;
But to climb back again, to retrace one's steps to the upper air –
There's the rub, the task.
– Virgil, The Aeneid (Book VI)