There isnt much value in incarcerating additional people simply for the sake of punishment/revenge. The other individuals are unlikely to cause future harm.
The idea is that everyone knowingly involved goes to jail. This way others presented with opportunity to go along with such fraud /know/ there is a very, very real consequence. Rather than "CEO could go down for this but what the hell, I'll go along with it."
It's not retribution, it's justice.
There's a YouTube channel called "Insider" and they have a series of videos called "how crime works" which are part interviews with past-criminals about how their particular scheme worked:
https://www.youtube.com/c/thisisinsider/videos
The videos all have extensive credits and one thing was that very few of the participants actually considered potential punishment/incarceration beforehand.
To this end, perception of the likelihood of being caught is far more important than the severity of punishment when it comes to deterrence.
People might scoff at mid-level fraud “doers” getting a slap on the wrist deal for their testimony, but the alternative where you go full scorched earth and fail to get a conviction would be terrible for the defrauded and the system beyond them. Capitalists (i.e. the owners of capital, the investor class, in this case) love a system where you don’t really have to quadruple check that a financial document is valid. Who can blame them? I certainly wouldn’t want to live in a world where something like FDIC doesn’t exist or where fraudsters can easily rip people off with low chances of jail time if/when the operation goes belly up.