I think that's because it's a perfect cube, which doesn't happen very often. But you can make it give you more supports by changing the support angle tolerance to 45 degrees or less in the Advanced Settings. But if it will work to let it lie flat, that's much easier.
Note that there is a size limit to printing surfaces flat on the build head, at which point you'll start to see the corners deforming upward. (This is due to the way the resin shrinks as it cures.)
For increased dimensional accuracy of square parts, you can also look into Pattern Mode. It's a little complicated to set up, but it can be useful if you need tight tolerances and your parts are square. (If you'll ultimately be making curved parts, there's no need to use it.)