If I had to guess it’s because most of the people writing those recipes have only been to New Orleans so they exposed to Creole thinking it’s Cajun. There’s usually not much of a difference between the two other than Cajun tends to be more on the savory side while Creole is more…sweet? Not sure the right word to use but they tend to get different flavors as they add more variety of vegetables to the dishes. If I remember my history right most Cajun dishes are taken from the Creole people but prepared using the Trinity since it’s the closest the old Cajuns could get to the French mirapoix. You can kinda boil it down to Cajun being French and Creole being a mix of Native American, Carribbean, and Spanish. There’s definitely some food that’s more common to Creole people than Cajun (pickled pigs feet for example) but pretty much all the food is shared amongst the two barring some differences in recipe and preparation.