Campden tablets need to be crushed before use. So why isn't Campden sold in powdered form?

Presumably this is either because:

1. The dosage required is very precise, and it would be very difficult to measure this out by weight or volume; or
2. Campden is unstable in powder form; or
3. [According to Wikipedia](https://en.wikipedia.org/wiki/Campden_tablet), eight Campden tablets are equivalent to ½ teaspoon of potassium metabisulphite. If this is the case, then perhaps Campden isn't *just* potassium metabisulphite? In which case perhaps the reason Campden is sold in tablets is because it contains a second ingredient which would otherwise separate from it?