I work in an automotive facility that uses injection molding and blow molding.
The basic reason you can't recast parts with higher quality plastics is because you need high temperature and pressure to inject the plastic. As was said previously you need special equipment to do this.
As for why the bootlegs are so cheap, it all comes down to quality. The quality of the toy and the quality of the molds they are using. Better quality is obviously more expensive. If you want a super accurate part then you would get a CNC cut steel mold. But for all we know the bootleg companies could be using poor quality aluminum molds which are a hell of a lot cheaper but do not produce as good a part. There are also other issues such as production volumes, overhead, design and development, etc. that all affect piece price.