That said, I’ll try playing devil’s advocate.
1. Some items are from areas now controlled by very different people to those for whom the item was originally important. Do modern descendants of the Spanish have any moral claim to Aztec or Inca artefacts, for example?
2. Some items have complex histories, where multiple competing countries could all claim it as part of their history. Where the item was found is not the end of the story. Items from Greece, Turkey, Cyprus for example.
3. By storing them in the British Museum, they are better maintained for future generations. Globally, more people have access to them right now.
Perhaps for those where the answer to all three of these is obvious, it’s indefensible that the British Museum still has the items. But there is quite a lot of stuff there where it’s not so obvious when you look into the details, though we should be putting a lot of effort into trying anyway.