If my spouse is living in another country and is a citizen of that country, why do I need to put an social security number for him to file my taxes?

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.