When you're using a knife and a ferrocerium rod, the sparks come from the rod, which is softer. The energy from cutting a chip off the rod ignites the chip. You need a sharp tool to actually do the cutting. A hacksaw blade is great because it has multiple cutting surfaces, so you get multiple sources of sparks. And the tips of the hacksaw teeth are hardened.

The same thing happens in reverse with a flint and steel- the flint is harder, so it cuts chips off of the steel. The softer the steel, the easier this is to do.