I think it's because there are no earlier references to the scarves, so "det" doesn't refer to anything specific.
"De er mine skjerf" would work if you were talking about the scarves and saying they're yours. In which case there would have to be an earlier reference to the scarves
A lot of neuter ('et') nouns, especially ones with only one syllable, do not add an ending in the indefinite plural. You can tell that it is plural here because the possessive pronoun 'mine' is plural.