Why does the crust of a freshly baked loaf of bread become less crunchy/hard over time?
It is not absorbing water from the atmosphere, rather, moisture is migrating from within the bread.  Basically, as soon as a loaf of bread is removed from the oven, moisture migration begins to happen.  It doesn't take long (hours-days) for that moisture to impact the crustiness of bread.  Moisture migration accelerates staling (which turns out to be more complicated than simply drying out, but that was not your question).  The good thing is that heat can effectively and temporarily delay or limit moisture migration.  Just reheat your bread in the oven or toaster, and it should regain much of its crunch.