[TheCoelacanth]

They also may have even more of a shortage of trained pilots than of fighters since they were willing to give a fighter to a random farm boy who happened to show up for a battle.

While the Empire can afford to throw away pilots, the Rebels need to give their pilots every chance to escape from a failed battle.

[sillywalk]

> a random farm boy who happened to show up for a battle.

To be fair Luke did "bullseye womprats in his T16", and his friend and rebel pilot Biggs could've vouched for him.

[science4sail]

> they were willing to give a fighter to a random farm boy who happened to show up for a battle.

My impression was that Luke's X-Wing was his reward for saving Leia - in the same scene we see Han leave the Rebels while carrying a sizeable amount of money.