What to take away from failed interviews when you don’t really know why you failed?

After every interview and hiring decision, I keep notes on what went wrong, what I could improve, and why I either moved forward or got rejected. I recently finished two onsite interviews where I walked away feeling genuinely good about my performance and how I handled the conversations. For one of them, I was honestly pretty confident I would get an offer.

Instead, both ended in rejection, or at least that is how I see it since one company completely ghosted me afterward.

What I am struggling with now is figuring out what I am supposed to learn from experiences like this. If I prepared well, communicated well, and left feeling positive, then what exactly caused the rejection? More importantly, how do you improve when you cannot even identify what went wrong?