Why some fungi are called as 'imperfect fungi'?

(a) The ‘imperfect fungi’ are kept in class deuteromycetes. They are called as imperfect because sexual reproduction is absent in these forms.
(b)They reproduce only by asexual spores called conidia. They have septate and multinucleate mycelium. e.g., Alternaria, Trichoderma, Colletotrichum.