Jack feels there's something missing in his life, but he doesn't know what it is. He's had this feeling for years and never known what it meant, and eventually learned to ignore it.
He has no way of knowing that one universe over, Daniel knew, instinctively, that he was meant for more than teaching English as a second language. Even if he did know, he would somehow doubt that it had cosmic significance. The guy has who knows how many degrees and ten times as many languages--of course he would feel he was meant for more.
The tape in the canopic jar is not the revelation to Jack that it was to that other Daniel, because the Jack who went back in time didn't say any of the things that were really important to him. That other Daniel saw the tape and thought, "That's it. That's the thing that's missing in my life."
Jack sees the tape and thinks, "Huh, when's the last time I actually caught a fish out of my pond?"
Five thousand years ago, or thereabouts, Daniel leaves Egypt to find himself.
Only, it's not himself he's looking for. It's his team. His family. He'll never find them, but he figures his chances are better out in the great, wide world than in Egypt.
Five years prior, by his count, Jack and Sam and Teal'c were all executed by Ra. Now Jack and Sam and Teal'c from an alternate timeline, an alternate universe, are here, but they aren't the same.
Teal'c has never seen his people freed. Has never watched cable TV. Has never helped his pretty neighbor flee the law. His wife is not dead, but nor is she free, and Teal'c can never return to liberate her and his son now.
Sam never joined the Air Force in the first place, which is proof that more changed in that universe than just the absence of the Stargate program. She's watched her father die of cancer and held his hand as he said, "I'm sorry, baby, I'm sorry I won't live to see you in space," while she smiled and cried and kept to herself that her new NASA position was as secretarial as the job she'd had at MIT.
Jack is close enough that it makes Daniel's teeth hurt. Jack is the guy who has lost his son and lost his self and stayed lost until the Air Force showed up on his doorstep with the opportunity of a lifetime. It almost makes no never mind that the Air Force showed up nine years later in his world than in Daniel's.
Daniel can bear how different Sam and Teal'c are. It is how similar Jack is that drives him away. It is how Jack can be so close the the man Daniel knew, the same temper, the same humor, and be sucking face with Sam like there isn't another soul in the world.
To be fair, she is the last soul from Jack's world.
That doesn't mean it's fair to Daniel.
Something like nine years ago from present day, in the neat and final timeline, Ra captures Daniel on Abydos and says, with loathing, "You." He punishes Daniel for crimes he has yet to commit--for crimes that the Daniel of this universe will never commit.
Daniel stays on Abydos after they have killed Ra not so much because he has found love with Sha'uri as because he is too broken to face a return to Earth. She nurses him as best she can, and he clings to her, but he does not fully heal, and when she is taken by Apophis, it seems to him that he may never be whole again.
He searches for her with SG1, but they are too late to save her. When she dies, Daniel feels as if a piece of himself has been irretrievably lost. Even the visions she puts in his head do not ease his pain.
He carries on, but he is broken inside, and fears to let anyone close enough to see how empty he is.
As Rodney McKay knows, and if he doesn't, he should, time travel doesn't work like that. You can't go back in time and change your own future. You can go back in time and create a new future, a new timeline, a new universe--but your old universe is still sitting there, unchanged. And if you never go back, which you usually don't, even if you go forward again, then you're just gone from that universe, and the people you've left behind are left assuming that you failed, and time travel doesn't work.
So there is a universe in which Gateship One goes through the stargate to Chu'lak and never comes back, and when Apophis's ha'tak vessels show up, Rodney McKay saves the world.
There is also a universe in which SG1 goes back in time to retrieve a ZPM from under Ra's nose, and are never seen again. There is no tape in a canopic jar in this universe, no tablet in an obscure dialect. There is no record of what became of them.
Eventually, they are declared MIA.
Sam had a brother she barely spoke to, whom General Hammond notifies. (It falls to General Hammond to do this because Jack is gone. There is a new head of the SGC, but they were General Hammond's kids. He wants to be the one to bear this burden.) Teal'c had his son, and General Hammond makes one of his rare trips offworld to break the news to Ry'ac. Jack had no living family, but his will makes mention of his ex-wife, so General Hammond goes to see her as well.
Daniel had no one left on this Earth but SG1. His grandfather is incommunicado with the aliens of the crystal skull. Most of his former colleagues in the academic community have been sucked into the Stargate program, and are either dead or on Atlantis. He spoke to Catherine every week, but she, of course, is newly dead as well. General Hammond has people sort through his things, his journals, his computers, partly to recover all of Daniel's projects for the program, partly in hopes of finding someone to tell.
General Hammond thinks, for a moment, that the someone has been found when a ring tumbles out of the pocket of Daniel's civilian clothes, left in his locker in the locker room. But then he looks inside the ring and reads the engraving, and knows that there is nothing he can do.
The ring says, "For my Spacemonkey."
If General Hammond thinks, disapprovingly, that it is indiscreet, he can only remind himself that it is the only suggestion of anything worth being discreet about that he has found.
He hopes that wherever and whenever his team is, they are happy and well.