The ballot drop boxes in Washington and Oregon both have fire suppression systems that are designed to activate when the temperature inside reaches a certain point, coating ballots inside with a fire-suppressing powder.
For unknown reasons, the system failed to prevent the destruction of hundreds of ballots in Vancouver, just across the Columbia River from Portland.
Why not just use post boxes? Less of a concentrated target because there’ll be a whole bunch of other letters in there too. At least, I don’t believe my country uses special boxes just for votes
You can drop it in the post but many people are paranoid about it being lost in the mail. The US also suffered a not unnoticeable degradation in the post service under Trump and his post master general pick who is still at his post because he didn’t resign and Biden didn’t fire him.
I still vote in person because of my own anxieties about my vote being counted.