simple...
overconfidance
It's human nature. If it's too easy we relax. Maybe is an evolutionary trait to conserve energy or something. So it's the leader's job to keep reminding everyone that it's not over and that stronger enemies are coming.
But in this case there wasn't really a leader. And being S-ranked hunters they were so used to being the strongest that once they realized how weak the foot soldiers were they let their guard down. Plus, I highly doubt being alert would be enough to save them. Otherwise they wouldn't have died so fast.
I ain't the leader type but I've never relaxed in a battle field. There could be a traitor in the midst waiting for you to out your guard down or a sniper hiding far away. Plus, being alert might have made little difference. One of them could have sense the danger before the healer died and warned the others to either run or something. I agree that they may not have been able to survive even if they were alert... but between 0% surviving and 1%, I'd pick the 1% even if the cost are arms and a leg.
Fair enough. Though in this specific case I believe they would have died no matter what. Because the attacker was too fast. Even knowing something was coming they still died. So yeah, I don't think running would have saved them. But MAYBE they would have had time to at least warn the other Japanese. Who at this rate will all die. I guess we'll have to wait and see if Goto has any chance.
In every war or battles I've seen, why is it that almost everyone's always putting their guard down/underestimate their opponents just coz they defeated the foot soldiers??? Unless everything's completely obliterated, assume that you're gonna die any second while you're in the battlefield. Simple common sense.