What do they have to lose?
If Putin can't stomach the loss of Crimea as well as a lost war in Main Ukraine, they may lose thousands of citizens and even more territory. If Russia goes TacNuke, Ukraine will lose a lot.
It's a question of how far to push Putin without triggering more than they can handle.
Russia is already trying to roll the whole country, and they're not going to just stop. They are trying to wipe Ukraine out and they just can't do it. What do you think their goal is?
So from Ukraine's perspective, why concede anything?
Or do you think Russia will be happy with JUST keeping Crimea and walking away from this? I doubt it.