Because that, too, was a largely mythical event?
There were several "fires" at Alexandria over the years, the most important probably being JuliusCaesar's warehouse mishap in 48 BC.
While stories of pagans, Christians, Hypatia, and flames are exciting, the real destroyer was fungus.
Papyrus scrolls rot away on their own in Alexandria. Any scroll not recopied roughly every 100 years would generally get moldy, crumble, and vanish. Which is what probably happened to most of the scrolls gathered by the Ptolemys.
The threats to our libraries are real -- but let's leave the fervid historical analogies to others.