Luke Skywalker grows up on a humble moisture farm on Tatooine. His father, Anakin Skywalker, has adopted the name Darth Vader and become a villanous tyrant. He never bothers to check up on a kid with his own unusual last name.
Why? Redditor scc10n8 has concocted a brilliant explanation. The entire plot of Star Wars Episode IV was a long con set up by Obi-Wan Kenobi:
I don't think Obi-Wan ever meant to confront Luke. I subscribe to the notion that the whole thing was an elaborate trap to lure Darth Vader there for a final battle to the death.
He takes Luke there as a child, doesn't even change his last name to anything other than Skywalker, and leaves him with Vader's mom's family on a sparsly populated planet so there's no over abundance of life to hide the kids force talent which is sure to manifest. Then he just sits there in the desert, waiting for Vader to show up so they can throw down on a hardcore force battle in a desert where there won't be much collateral or many opportunities to take hostages. Only he never shows.
Obi-Wan's plan doesn't work, though. Vader never shows up. So when he stumbles across Luke in the desert, he takes a different, even more insidious approach:
Sure Obi-Wan is old, but this kid is fresh. He's maliable. The Empire is sure to be tracking these droids. He just has to distract him for a while and the inevitable is sure to happen to his family. So what does he do? Start dropping hints about the kids dad. Tell him about him being a great jedi, a great pilot, both things the kid will need to off Vader, and all of it true. And as long as he's hearing what he wants to hear, has stars in his eyes, drop the bomb. Vader killed your dad.
From here Obi-Wan's got him, he's getting ready to somehow steer the kid to go check on his family when they seen the plume of black smoke on the horizon. A blasted out Sandcrawler, mana from heaven! After that the whole thing is dominoes. One after another gathering speed. And in a later showdown with Vader he can die with a smile on his face because he knows he gave it the first push.
-via Super Punch