Monk... Why do they suck?

[Gnaeus]

While there is no maximum baseline of competence, there is a minimum one. Things like Fighters and Monks are below the bar. Anything you use will turn them into a fine red mist. Things that can threaten the actually good classes will do this even more, but the problem is that the Fighter and the Monk aren't tall enough to go on this ride.

The bar for the ride is adjustable. Saying that fighters and monks can't kill anything, when clearly they can, is false on its face. A decent DM can compensate for a Fighter, Monk, Ninja, Healer party a LOT more easily than he can make challenging but not overwhelming encounters for a Wizard, Archivist, Druid, Beguiler party. The main reason for this is that the DM knows EXACTLY what the low-op party can do on a given day.

And threatening an optimized group doesn't require one round kills, though those will often happen anyways because it's D&D, and there's rockets flying around.

It doesn't require it, but it is a common result. If my group regularly breezes through CR+2-3 enemies, and I start throwing CR+4-5 enemies at them to compensate, it is likely that the attacks that they generate are one shot kills against characters 4-5 levels lower than they were expected to fight.