For drawings it is not common to use automated sequences for document naming. Drawing numbers have often specific ranges, stay the same while only the sheetnumber is changing, are renumbered to fil in gaps etc.
Most off all we must be able to comply to company standards. We are not able to introduces meaningless numbers. It will also lengthen the number and because of this it will not fit into available space in the titleblock.
Lots of reasons... Please enable the OK button in configuration when no sequence is added.
Because of the fact that manual input of a documentnumber is still possible it's better to check uniqueness ( as an option) when the properties are applied.