Because the Russia is committing a genocide in the territories it has occupied from Ukraine, and Ukrainians are fully aware that the same would be done in all of Ukraine if it was to surrender.
As long as Ukraine does not surrender, the Russia will keep bombing their cities and many will die.
If it surrenders, there will be much more Ukrainians dying than just that amount.
Not surrendering is a way for them to stay alive.
They know they cannot surrender without getting an extremely strong punishment from the Russia, so they won't.