I hate to defend bad writing, and I hate even more when fiction feels the need to retroactively try to explain things that don't need it, but here goes anyway.

In the novel Kenobi the Tatooine locals find out his last name and immediately start wondering if he's related to any of the other Kenobis they know in the area. So they do at least give a few paragraphs to pointing out that it's a somewhat common last name.

Then there's "Skywalker," where fans say oh it's a common last name that's why no one batted an eye at it. But I just assume that Luke went by Luke Lars up until the events of ANH.

There's probably something in canon that contradicts that, and the Kenobi novel is no longer canon either, but you know what? It's all made up and I don't care, I just picture it like I want to.