Because you need a drum anyway and they're the same price; about $35; plus the kit comes with four clutches. Have you even looked in the kit and compared the clutches?
I have been looking at kits but have not bought one yet. I also do not have the clutch packs fully disassembled yet. Just gathering info at this point.
If the HD drums cost the same and most kits come with 4 clutches, then I agree, that's an easy upgrade.
There is a transtar near me (Houston). I was thinking of talking with them to see if they recommend a certain brand of kit and replacement drum.