Some people are fighting back, mostly young men of somewhat courageous disposition. Also 1,2 million Ukrainians have already fled the country which far exceeds the number of Ukrainian combatants. I'm sure some of them would see surrender as a better outcome than loosing their homes and home country. I also heard stories about men between 18 and 60 who weren't even allowed to leave the country, so that number would be even higher if this demographic was given a free choice.