Because it's a Skyhopper, not an FTL-enabled ship. And you can see it in the background in the R2D2 hologram scene. But it shows that he has piloting experience, unlike Rey who jumps into the Millennium Falcon and is an expert pilot with absolutely no previous experience at all, then she knows how to fix it better than the person who has owned it for many years.
The novels deal with this, giving her familiarity with the Falcon itself, as well as with a wide range of ships on a flight simulator that she mastered to the point where apparently nothing the simulator could throw at her was beyond her capacity to handle. That doesn't help the movies--would have made sense to build in a little backstory, but it seemed like they just didn't want to slow down for anything like that.
To throw one more opinion into the pot-- I think this shows how presentation can be more important than actual deeds. I think the main difference between the two characters isn't what they accomplish, but how the story presents them. Luke feels like part of a group, and that he wouldn't get far without the others--he needs teaching about the Jedi, he needs to look elsewhere for a skilled pilot, Obi-Wan saves him early on, and Han saves him again at the end. He takes his turn to make other characters look cool. Whereas Rey didn't feel to me like she particularly needed the rest of the cast to be there for most of the movie--even when she needs Finn to shoot down the fighters, the scene is set up so she's doing the actual work. Her only real "failure" is that she can't 1v1 Kylo Ren in their first encounter. I think it comes back to that thing I mentioned earlier--Luke gets a lot (maybe an implausible amount) of stuff done, but has moments where he's uncool or weak, and that made his achievements feel more earned, because it establishes that this guy doesn't always succeed. Rey feels like the writers were worried about her coming across as less than 100% strong, so there's less tension about if she'll succeed--if she's in trouble, she'll probably just reveal a new skill to get herself out of the situation. It didn't make her unlikable or annoying for me, but it meant she left less of an impression.
Yes I feel you've hit the nail on the head. It's not so much what the character is like, it's what their passage through the story is like. If everything Luke did quickly worked a treat—either through luck or skill—he would not have been such a memorable character. We wouldn't have worried about him very much. If Rey had more problems she can't solve (without a lot of help), and more failures in general, she would be a much more memorable character ...without changing her personality at all. For me, it's not so much personality that makes a Mary Sue, it's whether or not they are given a smooth ride through the story. Nothing ever really goes wrong for a Mary Sue.
I think that an individual consumer's mileage on it can be a combination of plot and characterisation, as it is with other points about the story. I do think this is along the right lines. By struggling through the plot, the characters grow stronger in some form. Mary Sues do not have that growth, and as a result, they become boring to the consumer. When that happens, you've lost them.
Which doesn't mean a thing. You don't get to tell a story and tell people you'll fill in the details later. It's on the screen or on the page or it doesn't exist.
