History and culture. Historically becoming a commissioned officer meant you were form the aristocratic class. You had to pay for the commission but just having the money didn't mean you were accepted

You coukd have the money to pay hut be determined not to have a family of high enough standing to be allowed to purchase the commission

If you watch documentaries on top British boarding schools there is a running background theme on the military. Alumni who joined the military and became teachers at the school, teachers who were in the military and mention that their former military peers earn more than them, the military being seen as the fall back option for students who failed their exams.

These were just documentaries about the school and how they affect the kids but you still hear a lot about the military. It is just seen s a back up option for private school boys. And Historically tied to those institutions

In WW1 when soldiers signed up, the state school boys with no post secondary education became the rank and file, and the privately educated public school boys who also had no post secondary education became their officers

Historically and culturally private schools and the military in the UK are connected