Short answer is probably because there is no reinforcing at the junction of wall and ceiling, there should be at least a layer of scrim [this is a loose weave of hessian ribbon about 50.mm wide, or? a reinforcing purpose made tape. Lack of either of above will result in cracking.
Remedy? do not try to scrape and fill you will have a never ending task, fit a cove of choise polystyrene or plaster.
Would you expect this to be in every room? My initial worry was it was related to movement (in some cases you can lift the ceiling a little from the wall).
The ceiling plasterboard should have had a noggin to support the plasterboard around all of the ceiling perimeter, if you can lift areas of the ceiling plasterboard then the noggins have not been installed.
My original suggestion of fitting a cove, of some sort could assist in stopping the ceiling plasterboard from moving?
Must admit I have seen the issues you have in several rooms at the one time, the most "common" area is above the stairs from ground to first floor.
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