Look, this isn't her usual gig, alright? She's only gonna be happy once this job is done and she can go back to the surface.

Half the time Tilly doesn't know how she ends up in the dungeons in the first place; one task led to another until what's-his-name wanted whatever-the-artifact from somewhere underground. If she gets distracted while they're talking and accidentaly agrees, it'd be rude to go back on her word.