Why do prosecutors sometimes chase "weak" cases?

Question

From what I understand, the Criminal Division of the Internal Revenue Service is an exception to this question. Its rate of winning convictions is something like 80%, and about half the remaining cases are settled on terms favorable to the IRS. So it loses only a statistically inevitable "handful" of cases, where there might have been unexpected exonerating circumstances.

This varies in different parts of the United States, but from what I understand, prosecutors in some parts of the country sometimes pursue weak cases, usually against poor defendants. They have to prove "guilty" beyond a reasonable doubt," but it seems to me that the level of proven "guilt" in many of these cases is less than in the IRS cases. And it seems that most districts' prosecutions have higher rats of acquittal and/or wrongful convictions than the IRS.

What could be the reasons for this? Could it be that the IRS is a "national" organization while districts are "local" Does the IRS do a better job of picking "guilty" parties than district attorneys because they know that potential defendants have the money and attributes to defend themselves?

(From what I understand, the IRS rejects potentially meritorious cases that come below the 80% threshold, while some district attorneys seem to prosecute based on "probable cause," and get many acquittals, or wrongful convictions.)

Answer 1

The question is based upon a false premise. The IRS conviction rate is well below that for all federal crimes including IRS prosecutions (which was closer to 93% as of 2012 citing this source) and has continued to trend upward.

The IRS conviction rate, in contrast, is similar to that of typical state courts. According to the law review article written in the year 2000 and relying on older data (which probably understates current conviction rates in criminal cases as of 2019): "the conviction rate has averaged approximately 84% in Texas, 82% in California, 72% in New York, 67% in North Carolina, and 59% in Florida."

There are reasons that prosecutors bring weak cases, but the example of the IRS as an institution that is more likely to secure convictions when it is actually one of the worst at doing so at the federal level, is sufficiently far off in terms of the empirical reality that the question poses a poor foundation for discussing them.

Incidentally, the high rate of convictions in federal convictions in federal criminal cases is not due to the fact that federal felony cases must be screened by grand juries, while felony case in state courts must be screened by grand juries in only about half of U.S. states. Federal grand juries indict defendants in something in excess of 98% of the cases presented to them, while in state courts with grand juries, where conviction rates are lower than in the federal courts on average, indictment rates in state grand juries are also significantly lower than in the federal grand jury system. Also, many states with a grand jury indictment requirement for felony cases have lower conviction rates than states that do not have this requirement.