USPS drop boxes aren’t very secure.
Some thieves have been known to stuff the drop boxes with cardboard to prevent letters from dropping very far into the box.
Some people (not the OP of this string) are not careful. Attempting to mail a letter the day after a Monday holiday, letters were sticking out of the outside drop boxes at the Post Office. I took my letter inside. The outside drop boxes are only used when they aren’t full and there is at least one more pickup on that day.