I vaguely recall something about this being due to the shallow water table, but its pretty hard to believe the trench construction and regulatory hurdles couldn’t have been overcome during the 3 year lag before first test flight. Then again, Musk was pretty aggressive about pushing for that first tranche of test data so I can see why he’d risk what we have now witnessed: An environmental outcome worse than the EPA bureaucrats would have imagined: Spewing a hundred tons of concrete debris all over the wetlands. At least Musk got his data!
That said, I’ve never seen any satisfactory explanation for why Musk didn’t just build Truax’s Sea Dragon and be done with the whole mess – not just “stage 0” but also the Starship tile problem, which IIRC Truax said he would solve by rotisserie rotating the steel reentry vehicle. Maybe I misunderstood him but that’s what I recall him saying would solve the reentry problem without tiles.