Memes

Various reasons:

Some artifacts were acquired legitimately either as gifts or through purchase. So have no reason to be returned.

Some states are too unstable or corrupt to be able to return them, such as Syria or Egypt. And there was a case recently where France returned some artifical to an afircsn state just for the president (? King?) To keep them for himself.

Some artifacts don't have an easy place to return them too. Like take the kohinoor diamond, do you give it back to the Indian government? The Pakistani government? The Afgan government? The decendents of of the Maharaja that signed over possession to the Queen, or the descendents of the people he stole it from? Or the people that person stole it from? And so on and so on. And at this point it's more historically important to the UK than it is to any other country.

Which brings me to the next point, some artifacts are important because of their history after being taken. The Rosetta stone is a perfect example of this. When it was discovered by the French it was rubble being used to construct a crude wall. If the French didn't recognise it might br important it would have been lost to history. And if it wasn't translated by a French archaeologist after the British took it, then it would still be insignificant, as the are other identical stele in Egypt and its actual cobtents are pretty mundane and unimportant. Literally the only thing that makes the rosetta stone significant is its history AFTER it was taken from Egypt.

And finally some cases the artifacts are only their because that country got invaded. Like a lot of roman artifacts in the British museum were brought by invading Romans. I don't think anyone sensible thinks they should be returned to Italy right?