Pattern actually mentions that the Cryptics have tried resurrecting them:
"We have tried to restore them. It does not work. Mmmm. Perhaps if their knights still lived, something could be done . . ."
He says they would need the original knights to do it. Since a dead/mindless spren can't bond a new knight it's probably not possible to restore them. Of course there might be another way but i assume the Cryptics, who are apparently the scholars among the spren, have tried everything they thought might help.
Edit: Took too long to find the quote, maxal was faster :/