In my personal canon, many the pilots at the end are new pilots who’ve ranked up since most pilots didn’t come home. They were previously in training, but not ready to go out on the big mission without the possibility of being in the way more than helping out. Probably among the guys we see preparing the ships, gassing em up, ect. before the attack.
Also, the Rebels had more pilots than ready ships.
(and before “But Luke had just showed up!”, not everybody who joins the Rebellion has the experience/skill/confidence or someone to vouch for em.)