Carboxylic Acid Nomenclature
When identical groups are present such that neither can be preferred over the other then, such nomenclature is preferred to emphasise on the equivalence of the groups such as in the one we have here. We cannot name in such a way that includes the three groups together. So, we name it that way instead.

As such, the rules of IUPAC Nomenclature are themselves arbitrary therefore, it is just something that is the way it is. For more information you might like to see https://old.iupac.org/publications/books/principles/principles_of_nomenclature.pdf, Page 34 onwards.

Edit: I almost forgot that a compound can have more than one name, even by the IUPAC nomenclature rules. As such, the names used commonly as IUPAC names are the preferred ones.