We say this because CPGZ is just a container format, meaning it contains other files inside it - it's not meant to be directly converted to a format like XLS, PPT, MP3, etc.
M na-ekwu nke a n'ihi na CPGZ bụ ihe ngwugwu, nke pụtara na o nwere faịlụ ndị ọzọ n'ime ya - ọ pụtaghị na a ghaghị ịgbanwe ya n'ụzọ dị ka XLS , PPT , MP3 , wdg.