Unlike Crew Dragon, Starliner, Soyuz and Orion, Starship does not have a launch abort system. From what I hear, NASA now has an institutional allergy against crewed spacecraft without a launch abort system. It's been burned into the culture by the two Space Shuttle disasters.

A second Starship booster can be used to get SLS off the ground as Starship's payload.

I'm waiting for the SLS Heavy, with two starship boosters on the side instead of the SRBs.

Don't give them any ideas!

That's officially part of the plan, because otherwise it would be politically impossible -- but if those things get delayed, which seems likely, NASA could "temporarily" use some other, saner way of getting astronauts to lunar orbit. I suspect that's the real plan.