This is because you know this option does exist, and you know how to setup Mixxx properly.
We do not have a nice Setup Wizard that will guide our users, so better expect the initial setup does not go according to plan ;-) . Moreover, sometimes you`ll be away from the regular multichannel setup ( e.g. traveling), so be able to switch instantly is beneficial.

I think the split option should be visible in the UI, right next to the other Headphones control. Just slap a nice tooltip on the toggle button and users are likely to explore the option and put it to good use where applicable.