It's not just that they're pushovers, it's a completely one-sided affair. They only hear from the prosecution (the defendant is not present, let alone granted legal representation) and the prosecution is allowed extensive leeway in attempting to gain the indictment including offering evidence that would not be allowed in trial, such as hearsay [1]. And whether a case can be won or not
at trial is a secondary consideration for prosecutors. The vast majority of cases end up finishing in plea deals out of court. Above somebody mentioned 3%. The exact number will vary by state, but is invariably well below 10%.
So more important than whether a case can be won at trial is whether or not a defendant can be pressured into conceding. And in many things it's generally not hard. Imagine you think you have an 80% chance of acquittal at court, which would see you set free immediately. Yet losing at court would see you serve up to 10 years. And the prosecutor offers you 2 years + time served, which with early release means you'll be spending about a month in jail. Even though you are innocent and think there's an overwhelmingly good chance of being able to prove as such, you'd be a fool to do anything except accept the plea.
In some ways I wish plea bargains were not a thing. It'd massively reduce our arrest and imprisonment rate simply because we could not fulfill the constitutional requirement of a speedy trial with millions of people in the system for mostly irrelevant crimes, and it would also avoid this sort of 'loophole' of allowing prosecutors to score convictions even when the defendant felt he would have a good chance of defending himself at trial but is unable to do so due to risk:reward considerations.
[1] - https://en.wikipedia.org/wiki/Hearsay_in_United_States_law#A...