This was inherited from 2.7x behavior with groups, but in 2.8 collections are also used as 2.7x layers, which turns to be a problem when pasting back clipboard content, since it would instantiate collections instead of objects, quickly leading to tens of new collections in the viewlayer... Instead, we only copy selected objects from the 3DView now. On paste time, those will be added to the active collection. Last part of fixes related to T61670.