How about because it is the Year of our Lord 2022, the last monarch of the British Empire is dead, and humanity generally agrees now that theft through conquest is a bad idea, and Britain realizes that decolonization has to cost it something to mean something, and giving back a shiny rock on the Queen's hat which literally serves no purpose besides being a gaudy symbol of a shameful, spiteful legacy, really isn't much to ask, all things considered?

And also the other jewels stolen from South Africa. Or at the very least, don't keep them on the regalia.

> decolonization has to cost it something to mean something

Didn't they, you know, give up their empire? And invite their former colonies to colonize them, to the point that they are now a minority in their own capital?

If anything this should have taught them that, not only will it never be enough, but every act of atonement will count for nothing and be immediately forgotten and discredited.

The idea that the British left their colonies because they just gave them up is hilariously naive

Anything bad they did they did with full malicious desire and intent, and anything good they did is either ignored or, as I said, discredited ("they only did it because they were forced!")

If they give up the Koh-I-Noor to whomever plundered it before them, ten years later someone like you will be claiming that it is hilariously naive to believe they just gave it up.