[LeifCarrotson]

-21, -18, -15, -12, -9, -6, -3, 3, 6, 9, 12, 15, 18, 21 is 14 prefixes.

I think the number of people who could (correctly!) name even half of those is much smaller than you think.

[danbruc]

You are missing - 24, -2, -1, 1, 2, and 24 for a total of 20.

[cyphar]

±1 doesn't require a prefix, and while in Europe "deci" is used in common parlance for units of power ±2 it's not really used in scientific or engineering terminology globally. I'm also not sure what real quantity you're referring to which requires the power ±24 prefix (yes, the term "yottabyte" exists but no measurement of data capacity uses it).

[danbruc]

±2 are centi and hecto with centimetre and hectopascal probably being the widest used ones, ±1 are deci and deca with decibel probably being the widest used one. For deca I can also not think of any real usage.

[Avamander]

Yeah, but the number of people who actually need to use those units do know how to name them.