With Iran, there's not a whole bunch of similar material, the death count estimates vary greatly from source to source, and we've got an untrustworthy president beating a war drum which probably makes people a bit more skeptical.. Atrocity propaganda to persuade a democracy to enter a war is something attentive people will be familiar with; incubator babies being tossed on the floor, dissidents being fed feet first into industrial grinders, people remember these stories preceding other wars and remember that evidence for the claims never materialized. Then there's the whole geopolitical angle where the Trump administration in fact supports Israel and Iran happens to be one of Israel's most powerful regional opponents. There are plenty of reasons to temper feelings of certainty.