Here's why:
* The Imperium is vast: With billions of people spread across countless planets, keeping track of family lineages would be impossible.
* Space Marines are bred for war: Space Marines are genetically engineered and trained from a young age to be soldiers. Their individual identities are subsumed into their Chapter, which is their true family.
* Brotherhood: Space Marines see themselves as brothers-in-arms, bound by their shared purpose and experiences. They are identified primarily by their Chapter and sometimes a unique nickname or battle moniker.
So, instead of a last name, a Deathwatch member might be identified by:
* Chapter: "Brother Corvus of the Raven Guard"
* Battle Name: "Brother-Captain 'Iron Hand' of the Blood Angels"
* Rank: "Sergeant of the Deathwatch"
Let me know if you have any other questions about the Warhammer 40,000 universe!