There are also expensive high-grade SLC NAND flash chips available that offer significantly higher retention time than the cheaper commodity channel TLC NAND (i.e. 10 years versus 3 years).

In general, though, whether NAND or NOR, the fundamental way that flash works is by creating an isolated voltage charge for each bit (or several bits, for TLC), making it effectively a vast grid of very tiny batteries.

Like all batteries, no matter how well-stored, they will eventually leak energy to the point where the voltage levels change enough to matter.

Further, it's not enough to simply make power available to this grid since the refresh of the cells requires active management by a nand controller chip and its associated software stack.