Can I use my iPad as an external monitor with this feature? If not, why? When will I be able to? Why wasn't this already thought of?

No.

There are third-party programs that allow use of an iPad (or even an iPhone) as an external monitor via WiFi (not very speedy, though). I do not know which of them might work under Mavericks.

There is an iPad app called AirDisplay. It requires a WiFi connection, so the answer is yes you can use an iPad as a second monitor.