Maybe as a one off for yourself, it wouldnt be tooo bad, but for Phantom to offer it, I can see why they don't.
New mould for the bulkhead, work out how to make the heater work from the other side, which would be tricky as its air intake is on the left, not sure if they ever made any LHD MGFs?
Wiring loom would need modifying, numerous brackets would need new jigs making.
Also one major thing is the body on the drivers side is wider to enable more leg room for the pedals, is there sufficient room on the left for the pedals? not sure, but it would be a major task to modify it.
When you consider how many RHD ktis they've sold, I doubt it would have any commercial merit to put the R&D into a LHD option.