That’s because the vertices are shared between the planes - Unity requires separate vertices for each face so that the lighting “normal” (which way the face is pointing) points in different directions. There’s some way of exporting it like that, but I’m not quite sure what it is - I know people have had luck with subdividing, though that shouldn’t be necessary. It’s often a “smoothing” setting.
You can get the same effect with a shader, but you’re probably better off getting the normals right.
If you share the vertices then each corner effectively points out at an angle rather than in the direction of the face and the lighting ends up the way you have it.