how come they don’t show

If Uncle Owen had been the kind of guy who'd let a kid have a dog, and if there were any dogs on Tattooine, and if a dog wouldn't be immediately eaten by the native wildlife if it went along with Luke as he did his chores... then Luke might not have been the kind of guy who'd leave Tattooine the instant he got a ride.

Because that was Luke, someone who didn't care what the future held, he thought anything was better than Tattooine, because he didn't have a single happy memory to attach him to the place. (And that is why his girlfriend ended up on the cutting room floor.)