I'd state is slightly differently: when you contributed money to the IRA, you did not pay tax on that money. So if there is a loss, the IRS is not going to help you out on money that you never paid them tax on in the first place.
You won't find a 'deduction' in the normal sense of a tax return when you originally contributed the money, but there was a reduction from your gross wages on your pay stub to your Box 1 wages that are the basis of your tax return...
