Why is the taxable amount on SSA so low?

Up to 85% of Social Security Retirement/Disability/Survivors benefits becomes taxable when all your other income plus 1/2 your social security reaches:

• Married Filing Jointly - $32,000
• Single or Head of Household - $25,000
• Married Filing Separately - 0

To see the Social Security Benefits Calculation Worksheet  in Turbo Tax Online version you would have to save your return with all the worksheets to your computer.   Or if you are using the Desktop CD/Download Software you can switch to Forms Mode (click Forms in the upper right) and click on SS in the list on the left side.

Before filing,  You can preview the 1040 or print the whole return

https://ttlc.intuit.com/community/accessing/help/how-do-i-preview-my-turbotax-online-return-before-filing/00/26160