Why was Luke Skywalker not given the last name of his adoptive parents?

They were from the old country – Mr and Mrs Skiwokerhovnitzov—and they wanted to Anglicize the name to make him more normative in his new environment.

Han Solo’s parents did the same thing. His real name was Hans Heinrich Soliveitsky.

You think his name should be Luke Lars?

He’s a fictional character in a made up yarn. I believe It was decided early on that Luke Skywalker would move a whole lot more merch than Anonymous Punk
Kid.

Fun fact: In his first draft, Luke’s last name was Starkiller.