The US Army sometimes uses DRYAD. [1] It's not very good, but if you use a new code sheet every 6 hours and don't send much, it's OK.
The military views secrecy as a short-term issue. The most important secrets are "we're going to attack X at 0500 tomorrow". After tomorrow morning, it doesn't matter if the enemy can read the message. A cryptosystem only has to slow the enemy down enough that they can't react until it's too late. Classic Navy line: "Where the ship was last week is UNCLASSIFIED. Where the ship was yesterday is CONFIDENTIAL. Where the ship is now is SECRET. Where the ship will be tomorrow is TOP SECRET."
The military views secrecy as a short-term issue. The most important secrets are "we're going to attack X at 0500 tomorrow". After tomorrow morning, it doesn't matter if the enemy can read the message. A cryptosystem only has to slow the enemy down enough that they can't react until it's too late. Classic Navy line: "Where the ship was last week is UNCLASSIFIED. Where the ship was yesterday is CONFIDENTIAL. Where the ship is now is SECRET. Where the ship will be tomorrow is TOP SECRET."
[1] https://en.wikipedia.org/wiki/DRYAD