Because they have orderbooks 2 years (at least) into the future so know what demand is there - and they are demanding deposits for future orders.
It's easy to see if this is true. Look for news on new factories opening:
Micron: https://finance.yahoo.com/technology/ai/articles/micron-mu-p...
Samsung: https://www.kedglobal.com/korean-chipmakers/newsView/ked2026... (note this is doubling Samsung's memory production)
SK Hynix: https://finance.yahoo.com/sectors/technology/articles/sk-hyn...
How long does building a factory take?
If the demand grows with their production they can sell more units at the same price.
If demand goes down by a certain percentage, they sell more for less + they lost the investment into new factories.
It all is based on IFs and about personality, about "optimism" vs "pessimism"
I for one think that the AI bubble will "burst" at some point and I think that then there will be a lot of hardware to go by.
Time will be the judge of my abilities to replace the Oracle of Delphi.