You need to prove that they exist. Otherwise, single taxpayers could make up a fictitious spouse in order to lower their taxes. From the IRS:
Taxpayers who cannot obtain an SSN must apply for an ITIN if they file a U.S. tax return or are listed on a tax return as a spouse or dependent. These taxpayers must file Form W-7, Application for Individual Taxpayer Identification Number and supply documentation that will establish foreign status and true identity.
