Names like toluene are "retained names".
In chemistry, a retained name is a name for a chemical compound that is recommended for use by a system of chemical nomenclature (for example, IUPAC nomenclature), but that is not fully systematic. - Wikipedia
‘Retained names’ are traditional or common well-established names that may be used either as preferred IUPAC name or as an allowed alternatives in general nomenclature, for example, naphthalene, benzoic acid and pyridine. IUPAC's Blue book, P-12.3
Later in the same draft, we read
Toluene, xylene and mesitylene are specific parent hydrides that are composed of two components, one cyclic and the other acyclic and saturated. These names are retained due to a long and well established tradition. Toluene and xylene are preferred IUPAC names, but are not substitutable; toluene is substitutable under certain conditions for general nomenclature. P-22.1.3
Furthermore, anisole and toluene cannot be substituted by other
methoxy or methyl groups, respectively. P-220.127.116.11
- Names like toluene, anisole and xylene are retained names. They are preserved for historical reasons; however, their unsubstituted forms are PINs (Preferred IUPAC names).
- The name 3,5-diethyl toluene isn't acceptable according to the systematic IUPAC nomenclature.
- If you want to name the compound below, it's called 1,3-diethyl-5-methylbenzene.