If my taxable income (line 15 on 1040 SR) is $78,840, why is the tax amount only $2671 (line 16) of the tax table (married filing jointly) says it should be $9062?

If your gross income includes investment income, it might make sense.

Long-term capital gains and qualified dividends are subject to lower tax rates than ordinary income, with a maximum rate of 20%.

For tax years beginning in 2021, net capital gains and qualified dividends are taxed as follows:

• 0% if taxable income is $80,800 or less if married filing jointly or surviving spouse, $54,100 or less if head of household, $40,400 or less if single or married filing separately, and $2,700 or less for an estate or trust;
• 15% if taxable income is $501,600 or less if MFJ....
• 20% if taxable income is more than $501,600 if MFJ...

It depends on what kind of income you have.  There are like 7 different ways to figure the tax.

If you have capital gains or qualified dividends the tax is not taken from the tax table but is calculated separately from schedule D.  The tax will be calculated on the Qualified Dividends and Capital Gain Tax Worksheet.  It does not get filed with your return. 

In the online version you need to save your return as a pdf file and include all worksheets to see it.

For the Desktop version you can switch to Forms Mode and open the worksheet to see it.  Click Forms in the upper right (upper left for Mac) and look through the list and open the Qualified Dividends and Capital Gain Tax Worksheet.