Think about the convieniece in the first movie. The driods just happen to get picked by Jawas that stop first at the Lars homestead. The one homestead with a connection to Kenobi. Any other would have done as far as R2 is concerned because Luke want part of his plan, but Luke is crutial to GL’s script. Star Wars has a lot of convenient plot points like that. Usually we don’t think about them.
MST3K has called such things Plot Convenience Playhouse. 😛