My understanding is because the President can basically declassify anything at any time. He could literally Tweet out state secrets or tell them to a crowd if he wanted. Hell, he could email them to the Chinese and just say, "Yeah, I declassified them."
There are no laws against the President being fucking stupid and mishandling material. Because it was never an issue before and no one ever thought it ever would be. Plus the President is, ultimately, the final word on this sort of thing. You can't go over his head. If he wants to Tweet out the nuclear codes, legally there is nothing saying he couldn't.