Lingam is also formless representaion of the divine. It is a symbol for Shaivites who practice Shivadvaita.So instead of worshipping formless Shiva they worship in form of Linga which is closest representative of formlesness. Also for the same reason one cannot find the temple for Brahma becasue Brahma means emodiment of Universe and it is not acceptable in Hinduism to confine Brahma into an enclosure. On the other hand Vishnu is worshipped in various forms according to incarnations and attributes. Though for basically all are same in Hinduism Supreme is considered as both formless & inform(aakara & niraakara). Salvation is acheived by both ways Bhakti & Jnana.
