Because the apparently straight lines contradict the labels. As drawn, the unlabeled bottom vertices are clearly 90°, not 80° and 100°. We must either conclude that the labels are incorrect, or that the figure is not drawn to scale. Either way, it’s insoluble.
Why do the labeled angles prevent us from assuming straight lines?
Because the angles aren’t represented accurately. It could be that the two angles that look like they’re 90° add up to 180°, but they could also not
Because the apparently straight lines contradict the labels. As drawn, the unlabeled bottom vertices are clearly 90°, not 80° and 100°. We must either conclude that the labels are incorrect, or that the figure is not drawn to scale. Either way, it’s insoluble.