In the novelization, it is explained that when R2 was first built, he was actually conceived as a pilot droid. Specifically, he was meant to pilot escape capsules in emergencies. However, as he was transferred from owner to owner, he gradually lost interest in piloting anything in lieu of his actual duties, and over time he in fact forgot how to do it. So when the opportunity finally came up over Tatooine, he wasn’t able to do a very good job.
Haha! Thank you for that…! 😉