There are many reasons we do not use SD cards/flash memory devices for movies and TV:
SD cards have extrenely slow read rates compared to optical.
Optical discs are a much more reliable data storage format than flash memory devices. SD cards and other flash memory devices need to be used every few months, or the electric charge will wear out, causing the data to be unreadable. On the other hand, a professionally-pressed Blu-ray disc can theoretically last 50-100 years.
SD cards are more expensive to manufacture than optical discs.
Just out of curiosity, how much does it cost per month to run a Lemmy instance? Not that I’d want to do that myself, I’m just grateful for our admins that keep Lemmy up and running.