The reflection is generated by rendering into an environment map, which is updated according to the car's location. Usually, these maps only contain the "distant", static scenery, but not always.
The real limitation is self-reflection, as would be necessary in the interior shots, and would require ray-tracing / multiple or per-polygon maps in order to account for the close ranges and wide angles / parallax involved. Either that, or they have a pair of maps, one for the exterior (still visible from the interior) and one for the interior, which will be grossly inaccurate because of the closeness and parallax involved, but possibly tolerable. Of course, that requires more memory and some way of knowing which parts should receive interior reflections, and which parts the external - think open-top cars, areas near windows etc.
Basically, it's a deliberate performance decision that is by no means unique to GT.