In Switzerland this is a very common sight: locking mailboxes. All houses and all apartment buildings have them. When I arrived in the USA I was surprised that the majority of the mailboxes here are not lockable.
I even don’t know why people in Switzerland have only lockable mailboxes. It’s not that mail theft would be a daily threat in Switzerland, to the contrary. It’s just normal that you can lock your mailbox. Therefore mail theft is something that happens extremely rarely. Vandalism on the other hand is sadly quite common.