The answer I would give is that the SIM toolkit is included in the AOSP code for the Pixel Tablet, and I imagine that the GrapheneOS development team don't have a specific reason to remove it.
As to why it's included in the AOSP code for Pixel Tablet by Google, I can't speak on their behalf, and nor am I a developer.
If I had to hazard a guess, I imagine it might be simplify or streamline the monthly Pixel Bulletins, since then all the Pixel devices rely largely on the same code.
The telephony apps might also be tightly integrated into the OS, such that other system apps have dependencies on the telephony apps.
Again, I'm being rather speculative here. Probably the way to gain a concrete answer would be to contact the development engineers at Google who were originally responsible for porting AOSP to the Pixel Tablet before it was released.