So how come the Earth formed as a flat plane while all the other planets formed round?
The easiest way to imagine this is with a flow of some liquid, like water. Uninterrupted it is simple, predictable; insert any large object into that flow and it will still be predictable, but the nature of the flow changes thanks to the presence of what is there. The Earth formed first (as any flat surface would form: at the meeting between two opposed forces) and its existence altered what could happen around it, by altering the flow.
That's the simple version at least, and the more generally applicable one. It's easier than going into the intricacies.