> why isn't this done
It IS done with every new model release. Why do you think SpaceXAI bought Cursor? And then immediately had a huge jump in capability with their next model version?
It's just done in large batches for economic reasons.
> backprop fundamentally wants the entire data set in every pass
I'm genuinely unsure what you mean, it's not even possible to run backprop in this way?
No, I'd have to guess that new model releases have either new from scratch or continued pre-training. This is not the same as continual learning. Starting a new pre-training session is a dramatically different affair and involves utilizing the entire source data set in some fashion. Not just continued training on new data. At least from my understanding.
Bolting on new data to an existing model (fine-tuning) is precisely what gets you catastrophic forgetting.