Barely anything was left in the library by the time it burned down. It's quite a myth that all that much was lost at all. By the time of its destruction, most of its books and knowledge has been moved elsewhere due to decades of reduced funding and support.

Furthermore, the burning did not destroy the entire library. It was a gradual process over a long period of time. What destroyed far more of the books is a lack of care to copy them. Paper and papyrus don't last that long, so the only way to permanently maintain them was by copying them out again. Which is a lot of work.