Why are all sour cream cultures I've found for sale online labeled as direct set? Why can't I reuse like I do yogurt?
If you can't find a reusable sour cream starter, you can use buttermilk starter.  Some bloggers and biology/chemistry professors just use fresh active buttermilk as a starter rather than ordering some online.  If you look at the various labels and product pages, you will find that both the buttermilk and sour cream starters contain the same four cultures: Lactococcus lactis subsp. lactis, Lactococcus lactis subsp. cremoris, Lactococcus lactis biovar diacetylactis and Leuconostoc mesenteroides subsp. cremoris.
In the Q &amp; A for their buttermilk starter, culturesforhealth.com also provides this tip for using it for sour cream:


It is best to maintain it in regular whole milk, rather than reculturing cream long term.


On another page, Dr. Frankhauser goes on to say:


SOUR CREAM is produced by the same bacteria as buttermilk, but the starting milk product is pasteurized light cream. Bacteria are less numerous than in buttermilk.


This may be related to the lower concentrations of lactose found in unfermented milk products that contain higher percentages of fat. My best guess is that companies don't market the sour cream starters as heirloom or reusable because starting the new batch from the last batch may not reliably provide results that are as robust over time as their other products. If you just use milk for your starter, you can reuse it over time.