Why are insects Arachnids and crustaceans in the same phylum?

Insects, arachnids, and crustaceans are all classified under the phylum Arthropoda due to their shared characteristics, such as having an exoskeleton made of chitin, segmented bodies, and jointed appendages. This phylum is characterized by a high degree of diversity, with these groups adapting to various environments and lifestyles. All three groups also exhibit similar developmental processes and genetic traits, highlighting their evolutionary relationships.