I'd like for someone to explain to me why the Earth is not a sphere but everything else is a sphere?

If looking out into space we saw that the Sun was flat, the Moon was flat, other observable planets are flat; I could potentially see an argument that the earth "might" be flat (i.e. not considering any of the facts I learned in grade-school physics).

But since everything in space appears to be a sphere, why do you rule out the possibility that the earth is a sphere?