Why are arthropods invertebrates?

all arthropods are not insects because.. Arthropods aka the Phyla Arthropoda consists of 4 classes. (yr 1 university level here :D ) they are the Crustacea arachnida myriapoda and as you said the insecta. ok so these CLASSES are well defined by the characteristics the organisms of the class bear in common. hence one should not confuse the insect class with the phyla arthropods because one is a main category whilst one is a sub category which aid in defining the main category as a whole. hope this was helpful.