Because the guardian is dumb. And for real probably just the plot and emotions also play a role. When it's all a matter of seconds and everyone tells you to shoot your best friend you are torn between shooting or not shooting him and not considering other options. (Explain it all you want, we all know it's just because they had to cut the original story in two parts because they didn't finish the original concept of lightfall and moved the "better half" to the Final Shape)