And it's a fair point, but not exactly apples to oranges. It was "just" Zahi Hawass who made the formal request back in 2003. What if it was Sisi or Morsi or Mubarak who requested this at the nation state level. A request from a minister is not exactly equal to that of a head of state. I know this feels pedantic, but welcome to the world of policy, IR, and jurisprudence. The Kohinoor question is different from the Rosetta Stone as there is an additional Inter-Nation and Intra-National gray area that expands the complexity, which also expands the cost of litigation, which in turn is of questionable value when all the countries have higher priorities between each other, internally, and UK relations.

From an Egyptian viewpoint - a good example might be as follows:

What if an artifact was found in the Gaza Strip in 1955 and taken to the UK, who should the UK return it to? The Gaza Strip government, the West Bank government, the Israeli government, or the Egyptian government?

Edit: cannot reply below -

1. I'm pointing out a specific requirement in IR

2. The Gaza example above is an analogy for the Kohinoor example. I honestly don't care about the Egyptian case because I have no connection with that part of the world and I assume there are different considerations.