Why is my tax $0 when my only income is from investments? (1040 line 16)

I have already filed my 2020 taxes and am now planning for 2021. I will be retiring so will have no W-2 income after May 1. Ultimately I need to figure out what my quarterly estimated tax payments will need to be. As a starting point I made a copy of my 2020 TurboTax file and deleted the W-2, leaving only investment income, which includes interest, dividends, and short- and long-term capital gains. The Taxable Income on line 15 is a significant amount (five figures after taking the standard deduction) but the Tax amount calculated on line 16 is $0. It does not seem intuitively right that this amount should be $0. 

This amount is calculated on the Qualified Dividends and Capital Gains Tax Worksheet:

• Lines 5, 8, 12, 6, 17, 18, 20, 21, 22, 23 are all $0.
• Line 24 Figure the tax on the amount on line 1 is not $0 (four figures).
• Line 25, Tax on all taxable income, Enter the smaller of line 23 or 24 here is $0. The amount of line 23 is $0, so this one step makes sense.

However, the steps and arithmetic on this worksheet are absolutely impenetrable. It is impossible to understand the underlying logic/tax code.

Is it correct that someone with that kind of investment income pays $0 tax, or am I missing something else in building my What If scenario?