Why does the mere act of retrieving a memory strengthen it?

Retrieval is itself a memory-modifying event — reconstructing an answer alters and reinforces the memory you just pulled up.

A memory isn't a file you read without changing. Each time you successfully reconstruct it, you strengthen the retrieval route back to it, making it faster and more reliable to reach next time. This is the direct benefit of testing.

Passive review never exercises that route — you recognize the material, but you never practice finding it unaided. On exam day, finding it unaided is the whole task.