Usually, PU fabrication would involve moulds and plastic injection facilities, etc.
Custom bodykits are fibre because the shape can be hand-laid, and it's easy to wrap fibre and apply resin hardener over the whole thing.
That's why custom kits are fibre.. because they don't necessarily need a re-useable mould. PU, on the other hand, would need an accurate, re-useable and strong mould.