Nothing stops you from doing exactly that i.e. withdraw and THEN negotiate, because then it will be two states negotiating on how to come to terms with each other's existence. Rather than now, when it is one state mulling wether to allow another state to be created (and finding, not surprisingly, that it rather prefers that it didn't).
Syrian rebels threaten to shell government forces if prisoners' demands not met (Reuters)
from the article: Netanyahu mulls gestures toward Palestinians to keep peace talks going