I am expecting a 1099-SA from my HSA. Why does Turbo Tax say I have to form 5329-T (when it's available to file) instead of offering to use form 8889?

This form is used to report a penalty.  Either Turbotax thinks your contributions were not qualified, or your distributions were not used for qualifying medical expenses.  Make sure you run or re-run the interview for the 1099-SA and answer all the questions including about whether your expenses were qualified and about what kind of insurance you had.  

If TurboTax is indicating that you need Form 5329 for something regarding your HSA, it has to be related to contributions HSA, not distributions from the HSA.  The penalty for early, nonqualified distributions from an HSA is calculated on Form 8889, not on Form 5329.

Make sure that you have visited the HSA section in TurboTax and have answered the questions TurboTax asks to determine the amount that you are eligible to contribute to an HSA for 2018.