Angiofibroma bleeds excessively because:

Angiofibroma is a benign, vascular, locally invasive tumor. It has a fibrous stroma with irregular vascular channels, lined by endothelial cells without elastic membrane. Smooth muscle are often absent or incompletely present in it due to which it has tendency to bleed profusely. It is usually found in adolescent boys between ages of 10 and 17 years. Patients presents with symptoms of nasal obstruction and epistaxis in localized tumors. It usually originates in the posterolateral aspect of the nasal cavity. CT is the best method for diagnosing it.