Why the members of dass-Deuteromycetes are considered the fungi imperfecti?

Deuteromycetes is an artificial class of fungi which has been created to include all those fungi
(fungi imperfecti) in which sexual stage is either absent or not known. Their mycelium is usually septate. Coenocytic forms are not known. Clamp connections, typical of Basidiomycetes are absent.
It is believed that most members of Deuteromycetes are actually Ascomycetes in which sexual
reproduction is either absent or yet to be discovered.