>>280495757
Why? Because she doesn't want to be dealt bread crumbs by another Devil, especially another Horseman.
Shigga obviously deals behind her back in every scenario because she knows that Yoru is an insecure buffoon who is obsessed with monopolizing the "glory" of defeating Chainsaw Man for herself.
If Yoru knew that she was just being strung along in accord with Death's wishes, she'd obviously lose her shit.
Yoru makes it clear that her ambition is not to play second fiddle to other Devils, and that's pretty much why she's bashing her head against a wall against Primals right now.