I'm afraid you're incorrect there.
There are many, many, of our users who are using a standard IDE PC HDD in their PlayStation 2 to run Linux. We don't support doing it, but it appears to work for many people.
The reason you can't buy the discs individually is purely operational - it's a lot of cost and work to break up the kits, and we'd be left with a lot of spares if this happened. It'd make more sense to drop the price of the kit, which we've already done once this year.
Sarah Ewen, Sony Computer Entertainment Europe