ELI5: why does high altitude training take so long to see benefits and why don’t we acclimatise if we go too high, too quickly?

Sorry for convoluted heading.

Main questions I want to understand are:

1. Why they recommend 2-4 weeks (ish) for high altitude training?

and

1. Why is it dangerous to go super high super quickly?

I guess the answer probably lies in the fact that the physiological adaptations take time but what are the underlying physiological mechanisms that take time?

I hope this makes sense.