I can think of two very good reasons.
The British Royal family are not the brightest group of people, and couldn’t honestly get a job in the private sector.
After a decent time spent in the military they are ideally suited to act as arms salespeople, which they eagerly do all over the world.
It was surely a stroke of genius (in so far as those dimwits are capable of genius) on the part of the Royals to allow the dead Princess Diana to campaign against landmines. Not only do they appear to be campaigning for world peace, but they can make lots of friends and money all round selling landmines to replace those dug up.
A uniquely royal vision of recycling.