I always got the impression that Rey’s background was carefully planned to fully equip her with a standard set of Star Wars skills so she could be a flexible protagonist right off the bat. Being a scavenger who’s spent years with criminals and lowlifes on a pretty rough planet makes her athletic, practiced with melee weapons, a skilled mechanic with a knowledge of old Imperial and Rebellion stuff that she’s salvaged, resourceful, able to fly a speeder (and other vehicles she may have had a chance to use on Jakku), etc. I felt this was intentional to avoid having her spend a whole movie being a confused farmboy or a child like our previous protagonists. Disney wanted TFA to make a bang all by itself, not just set up the next two movies, and they knew people expected lightsaber fights and force powers.