Why can’t I claim my spouse as a dependent when she couldn’t work

[xmasbaby0]

A spouse can never be claimed as a dependent.  File a joint return.

Married Filing Jointly is usually better, even if one spouse had little or no income. When you file a joint return, you and your spouse will get the married filing jointly standard deduction of $27,700 (+$1500 for each spouse 65 or older)  You are eligible for more credits including education credits, earned income credit, child and dependent care credit, and a larger income limit to receive the child tax credit. 

[DawnC]

A spouse is never a dependent.   If you are filing a joint return, both you and your spouse are listed as taxpayers.   If you file jointly, you receive a double sized standard deduction.