I think it's mainly because some see a punishment as revenge and others as correction.
But, as user publius_0xf3 is showing, revenge does not work. The victims don't get their money back.
If this sentence is used as correction I also think it does not work. Would such a correction really take 25 years? His life is over. I don't see how such a long time is helpful to him, to his victims and to society.
SBF probably assumed that if he got caught nothing would happen because he's witnessed that historically.
Society is not necessary interested in helping him. Ultimately he's not that important.
What is important is maintaining the rule of law and preserving faith in the justice system.
Treating it as a correction feels like a lie that polite society tells itself in order to absolve itself of the distaste of knowingly harming someone. We shouldn't pretend we can "re-educate" anyone. We can merely provide opportunities for self improvement, but we can't actively "correct" them.
On the other hand, treating punishment as revenge is unhealthy too. It's too easy to get carried away and it's even easier to get carried away by perverse incentives (gestures broadly at US incarceration rates). Two wrongs don't make a right, as they say.
So then how should society decide what punishment is fair? I believe the punishment should be as harsh as an elected judge feels is necessary for the perpetrator to think, "it wasn't worth this"—and not a bit more.
Isn't that using punishment as a deterrent? It's easy to see it that way, but no. That would make punishment impersonal again— unbinding it from the specific person, place, and circumstance that we should elect judges to consider carefully and compassionately. In other words, when one says, "the perpetrator should be punished {this much} to deter the others", then the perpetrator becomes a pawn, not a person.
All that leads me to believe that: the purpose of a punishment should be to inflict a harm equal to the perceived personal benefit of the perpetrator's crime, as an enforcement action of the social contract between the perpetrator and society.
I mean, I don't know this SBF guy, but do you think he thought there would be no punishment if he got caught stealing 8 billion dollars? I think his plan was not to get caught.
SBF caused an incredible amount of irreparable harm with his actions, which almost certainly has resulted in suicides. He deserves this punishment.