It’s only silly now in retrospect when the PT tells us Obi-wan is such a dummy that he’s basically saying “…only a master of evil, Lord”
The word “lord” is a weird case where you wouldn’t really use it in that way (more likely “my lord”). I always saw Darth as a title used more like “general” or “master” or “count,” where people refer to them by just their title all the time. I’m not a linguist so I apologize if I’m not explaining this sufficiently, though it should be easy to understand.
Probably worth mentioning that people refer to Kylo Ren as “Ren,” even though that is seemingly a title too.