Not completely fair comparison as 'pubs' (public houses) don't really match completely well against american 'bars'.

Because space was at premium in many UK cities, their houses were typically built without much lounge / living room space. Therefore, in neighborhoods it became common to dedicate one of the houses on the street as the 'public house'. They were used to host get togethers and since there was typically one on every street you would do your socializing in the pub rather than having someone come into your house. Its common to find whole families from 5 to 85 meet in the pub to visit with other families.

This doesn't totally map well to the concept of american bars, which are typically adult-only and more exclusively geared towards drinking and less about a neighborhood socialization center.