Great question. A big part of it probably has to do with culture and economics. Culturally, it seems that the early Anglo Saxons weren't interested in annexation as much as they were with subjugation (ie, becoming overkings). And economically, full annexation would have been really difficult due to the fact that taxation over large portions of land would have been really difficult since taxes were often in the form of food and drink. So forcing your neighbors to provide tribute, while leaving the hard work of actual tax collection and governing, was probably preferable.