I managed to find a workaround by using filter "contains any of" on the dimension and pasting in all values, I then created a segment from the filtered results. It would still be good to understand why some dimensions are restricted to only basic operators though.

Thanks @PratheepArunRaj Did you ever find out if it can be done?

So I'm trying to build a segment using the Region dimension to group them together to create wider geographical regions. Usually I would use the "equals any of" or "contains any of" operators and add in the list but I am only able to select standard operators: equals, does not equal, exists, does no...