How come the Doctor and Clara will never be able to see each other again?

The Doctor and Clara always had fun times together but their time came to a end in Face The Raven. After their last goodbyes, the Doctor goes 4.5 billion years in the confession dial to save Clara and tells her what has happend to her:

DOCTOR: Your heartbeat. Your physical processes have been time looped. Frozen between one heartbeat and the next. Even your breathing is just a habit. You don't need it.

They they argue about whose memory will be erased:

DOCTOR: It won't hurt, it'll be nothing. You'll just pass out for a moment.

CLARA: And then?

DOCTOR: When you wake, you'll be fine.

CLARA: But

DOCTOR: Clara, just listen to me.

CLARA: Just say it. Say it. Come on. Tell me.

DOCTOR: When you wake up, you will have forgotten me. You'll have forgotten we ever even met.

CLARA: And why would I want that?

DOCTOR: Because it's the only way. That stuff in your head, the image of me, they could use it to find you.

This becomes important for both of them:

DOCTOR: I'm trying to keep you safe.

CLARA: Why? Nobody's ever safe. I've never asked you for that, ever. These have been the best years of my life, and they are mine. Tomorrow is promised to no one, Doctor, but I insist upon my past. I am entitled to that. It's mine.

DOCTOR: You and me together. Look how far I went, for fear of losing you. This has to stop. One of us has to go

Then the doctor uses it and his memory of Clara is erased. Then he is outside, and goes to a diner to talk to a waitress about Clara. Then they both go off on new adventures in their TARDISes never to see each other again, which made me think: how come the doctor and Clara will never be able to see each other again?