Recent raid gear can not be turned into seals, for your little reason at the end of the OP. This is because it's gear that will last quite a long time. Some Dun Scaith can even be BiS beating 270 at this time depending on what item it is.

Dungeon gear is very quickly replaced, while seals and desynths have real long term effects. Why is something replaceable in under a week better than a long term goal/item?