There are actually direct RER B trains between the airport and gare du Nord in central Paris and other RER B trains that stops at all the stations between the airport and gare du Nord...

There is now a project to create another direct line between the airport and Gare de l'Est (and shutting down the direct trains of line B at the same time), the CDG express (https://en.wikipedia.org/wiki/CDG_Express and more info in French https://fr.wikipedia.org/wiki/CDG_Express). The project was indeed stopped in 2011 butis now alive again, and very likely to be made. Se also http://www.cdgexpress.com