Because that is what the instructions tell you to do?

Lines 9, 10, and 11
Use lines 9, 10, and 11 for business use of the home expenses that would have been deductible as a personal expense if you had not used your home for business. These expenses include certain casualty losses, mortgage interest, mortgage insurance premiums, and real estate taxes.
Taxpayers claiming the standard deduction. If you claim the standard deduction, you will not include any mortgage interest, mortgage insurance premiums, or real estate taxes on lines 10
and 11; instead, you will claim the entire business use of the
home portion of those expenses using lines 16 and 17.