TOM BIIIIIIIIIIIIIIIIIIIIIISHOP!!! COME 'ERE!!
I'm not saying it's an illusion, I'm saying it may LOOK like it's flat without actually being flat. Things aren't always what they look like, and that's not because they're an illusion, but because one's brain interprets it that way.
When you look at a convex mirror, the image of an object appears to be behind the mirror. This is because your brain interprets light as always traveling in a straight line, however if you were to place and opaque object behind the mirror where the object appears to be, nothing will change in the mirror. This is called a virtual image, and it is not an illusion.
Of course looking into a convex mirror and looking at the Earth are 2 very different things. But I find that saying that the Earth is flat because it looks that way is rediculous. It's like saying the Sun is about 2 cm in diameter, because if you hold a quarter up, you see that the Sun and the quarter are the same size. That's because it's so far away. That's because the Earth is so big.