Its quite likely the infinite earth always existed. Really this is a silly question - nobody knows how it form and ever could.
I second this notion, OP's question is a bit hard to answer from the context of a FE's perspective. Nay, I think impossible.
Its like asking "why does god exist?" The question itself is a bit counter-intuitive. I do however think it does have merit if one is trying to understand a flat earth.
The best answer I can give, would be that is was formed by some being, or beings, with knowledge and power far beyond our comprehension, the tools necessary to create such a place would be very difficult for a normal human to comprehend, and therefore, very hard to explain how it was all put together in the first place. However, it must have been really fucking hard. My hats off to whomever did it, if the FE's turn to be correct.