Fuck

In L4D making different categories for the different difficulties would be pointless, for various reasons:

1) running it on Easy would be impossible because you can't incap the bots; on normal it would take too long as well
2) besides the finale, every fight should be avoided whenever possible. The only damage that can be expected is from commons (or, in L4D2, spitters too), and it would be stupid to force someone to restart only because the director has been quite bad and spawned them in a bad place
3) healing can be done with almost no time loss thanks to the end-level glitch, so as long as the runner manages to keep above 40 HP damage is irrelevant
4) tanks on Expert with bots are way too time consuming, in my opinion

There are lots of more valid reasons, which I'm sure someone else will point out, since I can't think of any more right now.