system
Given the following post, respond as if you're commenting on Reddit:
human
Eli5: why isn't the british army called the "royal army"

Royal Navy, Royal Airforce, even Royal Mail.

Why doesn't the army get to be royal?
gpt
Historical reasons. The army is allowed to exist by order of the parliament, whereas the navy was personally controlled and raised by the monarch.  The army exists, legally, because parliament allows it to. 

The distinction is less relevant today than once it was, obviously.