I've already explained; prosecutors just don't do it that way.
1) Justice Department policy is to indict for the most serious provable crime, and not plead down to lesser charges. Not doing so suggests that there isn't the evidence to do so. In which case, what could the person in question possibly have to offer if he hasn't admitted to anything more?
2) Indicting for the most serious crime gives more leverage over the person you're trying to convince to cooperate, because you'll have him in a position where he has much more to lose and much more to gain cooperating. Indicting for a lesser crime front-loads the benefit of cooperating. Why give him the goods -- the lesser charge, the lesser sentence --
before he cooperates? Only when faced with a serious penalty is there motive for cooperating. A guy facing 20 years off the bat is more likely to sing than a guy facing 5, or none.
Are you more likely to cooperate with someone who has a gun pointed directly at you, or someone who says that he'll go get the gun you've never seen if you don't?
3) Indicting for the most serious crime makes for a more credible witness.
4) Merely indicting for a false statement makes the witness a lot less credible, because, well, he makes false statements, and there's little penalty for him if he reneges on a cooperation deal.
5) Indicting for the most serious crime brings more weight against the "bigger fish," because it establishes the serious crime.