Why do game characters still forget everything?

I’ve been thinking a lot about how NPCs work across game sessions.

You can spend hours interacting with a character, make choices that should matter, build a relationship, unlock things together — then start another session and most of that history basically disappears.

I’m building 4BTD (Beyond The Dialogue) around this problem: persistent memory and identity for game characters, so they can remember interactions, relationships, achievements, inventory, and important events beyond a single session.

Still early, and I’m curious how other builders see this.

If you were building a game with persistent NPCs, what would you want them to remember?