I think that there’s a line of dialogue in Supergirl season 1 episode 15 Solitude that might explain why we’ve never seen an earth 1 Supergirl in The Flash and Arrow
So I am rewatching Supergirl on Netflix and like the title suggests I’m at Supergirl season 1 episode 15 Solitude after Winn stops Indigo from choking him to death Indigo says the fallowing to Supergirl “You. You wouldn’t even be here if it wasn’t for me. How do you think Fort Rozz escaped the Phantom Zone? For decades, I searched the Phantom Zone with my mind. Looking for a way out. And then... I found you (Supergirl). I activated your pod. I linked it to the prison. I'm the reason you made it to Earth. You would still be sleeping in that timeless void, if it wasn't for me.” So what if in the universe that The Flash, Arrow, and Batwoman took place on before Crisis On infinite Earths happened and years before any of the shows but DC Legends (technically) happened when Earth-1 Kara was sent off Krypton and took a very similar path as our Kara (originally Earth-38 and Earth-Prime) the only difference is that Earth-1 Kara didn’t have an Indigo to activate her pod taking her pod and Fort Rozz out of the Phantom Zone