So something that just occurred to me and it makes so much sense for why, is why didnt Ozpin revive Summer?
Now ignoring if Summer was as important as people claim there is a few clues.
Yang said in Burning the candle
It was tough. Ruby was really torn up, but... I think she was still too young to really get what was going on, y'know?
Ruby was too young to really get it so that says young.
Currently Ruby is 15 to 16
Leo had this to say on the Spring Maiden in Welcome to Haven
She was determined, at first, when she inherited her powers, but the weight of responsibility proved to be too much for the child. She... ran. Abandoned her training, everyone. That was over a decade ago. There's no telling where she could be now.
So over a decade ago.
So Ruby 16 minus lets say 12 years for over the decade would place her at 4 or so. I dont think a 4 year old would understand death completely.
So if we take the idea that the Relic of Knowledge is required for a revival the reason why Ozpin didnt revive Summer is because he couldnt he didnt have access to the vault with the Spring Missing.
It would also explain why Salem couldnt find the Relic of choice. Ozpin would have realized that with the Maidens possibly being taken or just plain running and them being the only key to the Vaults locking powerful magical items he may or may not need a different solution might be required.
Its also brilliant because it means Salem will focus on the Vaults thinking the relics are still there when they arent at all.
So in short what I think might explain is that Ozpin because of Spring running COULDNT revive Summer since the Magical item he needed was locked behind what only spring could open.