Why? Because post processing is equally important as a skill, and relying on others to do the work is akin to letting them also take your light readings, or composing the shot for you. It just is.
We're not talking about plug-ins here.. like Silver Efex, that while they have presets, also offer full manual control.. these are actions... click and go. You CAN go into the actions pallet and reverse engineer them, but if you're going to do that, why not just create your own actions.. it's not particularly difficult.