lets just say speedforce gives flash a lot of bullshit he can do, though if you want one of the most straightforward reasons, its due to being fast enough to timetravel.
It does come with its flaws though. They just aren't brought up a lot.
Flashs speed is essentially a deus ex machina meaning there's a lot of stuff that he's done that's absolutely insane Iif you account for the actual speed to do it. Specifically, as Death Battle mentions, his 6 billion MPH speed.