Fogerty has stated that the song is about being left and not being able to learn much as a child.
"Someday Never Comes" is simply a song about my parents undergoing a divorce when I was a child and me not knowing many things. When my dad left me, he told me to be a man and someday I would understand everything. Now, I'm here basically repeating the same thing really. I had a son in 1966 and I went away when he was five years old or so and again told him "someday" he would understand everything. Really, all kids ask questions like "Daddy, when are we going fishing?" and parents always answer with "someday", but in reality someday never comes and kids never learn what they're supposed to learn. - John Fogerty 1973
When I wrote this song, my life was pretty chaotic. I knew my marriage was going to break up. My band was falling apart. I was beginning to sense the darkness that was Fantasy Records. This song was inspired by my parents' divorce when I was a young boy and the effect it had on me. At the time, they told me, "Someday, you'll understand." The truth of this is that you never do and I found myself facing this as a parent. The irony was painful and inescapable. - John Fogerty, liner notes to Wrote a Song for Everyone, Vanguard Records, 2013