There are multiple explanations. We care more about people than ideas, but we don't care zero about ideas. Especially if a startup is passionately committed to what seems a bad idea; we'll usually reject them no matter how good the people are.

When people reapply it's often with different teams. I think all 3 cases I mentioned did.

People do change somewhat between applications. Someone who's only 25 gets 4% more experience each year.

And also, as we say in the email when we turn people down, we're often simply wrong.