[paulrouget]

Because measurements confirm a homogeneous and isotopic universe. A spherical universe would imply a special point, the center, which would go against these cosmological principals.

[x3n0ph3n3]

Any non-infinite universe would imply a center point, and the cosmological principle may not be correct [1].

1. https://www.youtube.com/watch?v=WBfeKz1SG0kt=2m5s

[vlovich123]

Could be a universe that folds in on itself in multidimensional space so that every point looks like the center. But it almost certainly isn’t a 3d sphere.