Human brain's "pre-training" is evolution cramming way too much structure into it. It "learns from scratch" the way it does because it doesn't actually learn from scratch.

I've seen plenty of wacky test-time training things used in ML nowadays, which is probably the closest to how the human brain learns. None are stable enough to go into the frontier LLMs, where in-context learning still reigns supreme. In-context learning is a "good enough" continuous learning approximatation, it seems.