Thing is, if they time travel back to fix it every time any Federation citizen dies in a preventable manner, society collapses as it entirely consumes itself focusing entirely on constant timetravel, possibly billions of times a day, until Federation (and Federation-adjacent) day-to-day continuity just unravels.
And if they save it for just “those of us in the family”, that’s cronyism, and suddenly our heroes are deeply corrupt. (In a few canonical cases, they kind of already are. I love Sulu, but his arbitrary attempt to rescue Kirk and Bones from Rura Penthe against orders could have blown up what was left of Federation/Klingon relations at that point, and did get one of his crew — who in my head canon is one of the Valtrane twins both serving on Excelsior — killed. With his brother later forced to stand there in a bridge crew tableau and accept it.)*
(Likewise Admiral Janeway, but it’s not out of character for Janeway — who I also like — to not give a sh—.)
(*Yeah, I know — but having two Valtranes present nicely solves the did-he-or-didn’t-he problem, even if the situation then further problematizes Sulu. It also leaves a nice puzzle for an imaginary Cardassian historical mystery novel about the situation—Who really died, Dmitri or Masoud?!)