Why is "pen" of "submarine pen" not in the dictionary?

I saw this word a long time ago while playing red alert https://cnc.fandom.com/wiki/Submarine_pen_(Red_Alert_1) Since then i had forgotten about it, until a watched a youtube video on u-boats https://www.youtube.com/watch?v=omn6VK64t9Q Wikipedia has it too https://en.wikipedia.org/wiki/Submarine_pen "pen" seems to be a separate word but i could not find any definitions online that refer to something with ships. Could someone give a definition and explain why it isn't in the dictionary?

In the Merriam Webster online dictionary the definition of pen includes;

Noun 3: a protected dock or slip for a submarine

Thanks for your question which has educated me on the use of the word.