We sterilize plenty of other common tools like scalpels so that doesn't seem like a valid reason. Obviously the disposable design is not even an adequate solution to the risk of cross contamination. I would imagine if it were a real concern you could easily add something like a color changing strip that would indicate whether the needle has been autoclaved since its last use without rendering it useless.
They can "survive" autoclave cycles that render other pathogens dead/inactive, but there do exist autoclave cycles that seem to pretty reliably inactivate prions.