Skywalker wasn’t a common last name.
Vader/Anakin nearly killed Padme with a force choke. she was still alive though after but lost her will to live.
Obi wan took her to a secret base where she had a c-section after it was confirmed she was dying. Then it was discovered that Padme was pregnant with twins. Everybody close knew she was pregnant but nobody knew they were twins. So, Yoda and Obi decided to make padme appear as if she was still pregnant when she died and Only change Leia’s last name and not Luke’s. Obi wan was also tasked to watch over Luke and make sure the Sith doesn’t find him.
But Vader wasn’t seeing his children out by the time A New Hope started
So they didn’t need to make him Luke Lars.
But you would know that if you actually followed this crazy ass lore