Why the UK have fraternities?

The UK doesn't have fraternities in the same way that the United States does. While there are some student societies and clubs that might share some superficial similarities (social events, membership requirements), they are fundamentally different. American fraternities (and sororities) are typically large, national organizations with a strong emphasis on Greek letters, rituals, and often a residential component (living in a fraternity house).

The UK has a much wider variety of student societies and clubs, many of which are based around specific interests, activities, or academic disciplines. Some might have social aspects, including events and gatherings, but they generally lack the formalized structure, extensive national networks, and residential elements characteristic of US fraternities. There is no equivalent system of "Greek life." The UK's student social scene is more decentralized and diverse.