>So you'd expect a relatively high conviction rate just based on the fact that cases where conviction is unlikely are filtered out before they ever get to trial.
Except non-prosecution is a tiny percentage of cases where no trial is held. Only 2% of Federal criminal cases actually go to trial[0], and 97% end with the defendant pleading to a lesser charge.
Overall, something like 90-95% of all criminal cases in the US end with a plea bargain[1]
Usually, the prosecutor uses the threat of jail or prison (for more minor offenses) or the threat of lengthy (15 years? Life?) for more serious ones to force a defendant to plead guilty to a lesser charge. And this is true even if the defendant is innocent of the crime charged.
And when someone is poor and can't afford bail, prosecutors can exert even more pressure to force a plea bargain[2], as the defendant (whether or not they actually committed a crime) will sit in jail either until they agree to plead guilty to something or go to trial.
How long would you keep your job if you were sitting in jail? How long before you couldn't pay your rent/mortgage without income from that job? Or even worse, if you're a single parent, how long could you retain custody of your children?
And it just gets worse. Should you plead guilty, you now have a criminal record and most employers won't hire you.
And remember, these are people who haven't been convicted of a crime, and no court or jury has heard their case. Yet many people's lives are ruined just because they're poor and can't afford to pay bail.
Edit: Fixed percentage of overall cases which end with a plea bargain.
[0] https://www.pewresearch.org/fact-tank/2019/06/11/only-2-of-f...
[1] https://bja.ojp.gov/sites/g/files/xyckuh186/files/media/docu... [PDF]
[2] https://bja.ojp.gov/sites/g/files/xyckuh186/files/media/docu...