If russia does not recognize the sovereignty of Ukraine, their "idea" of "negotiation" is for Ukraine to submit, and there cannot be meaningful negotiation with such a bad-faith actor, it is a waste of time and energy and resources... but sadly the charade must be performed at some level, for the "benefit" of international eyes, even when many of them are russian "useful idiots".
In such circumstances, Ukraine understands that it is essential that they not cede an inch.
EDIT: I want to put the severity of Ukraine's situation into clear terms here.
russia stole their land, and any negotiation is an attempt by russia to legalize their theft. Now add to that how russia terrorizes and brutalizes everyone and everything they touch, and the moral imperative to NOT negotiate becomes overwhelming.