The reason? See my comment from 7 years ago! - https://news.ycombinator.com/item?id=21351115
The younger generation in particular need to know of these excellent resources for Science/Mathematics.
Another good publisher of old/classic Science/Mathematics books is "Dover Publications" - https://store.doverpublications.com/
Checkout for example, the books by Richard Hamming, in particular; "Methods of Mathematics Applied to Calculus, Probability, and Statistics" - https://store.doverpublications.com/products/9780486439457 (highly relevant to today's needs).
Also checkout the book series (mathematics via correspondence for high-school students) by I.M.Gelfand - https://store.doverpublications.com/search?type=product&q=I....