Of course they're not running as independents (except for Sanders), but that's not what you said:
"More because they hold the power and control the process. Like, who gets in debates and who doesn't. Or all the rules in Congress where the majority party gets committee Sears etc.
One thing DS and RS can agree on is to keep everyone else out."
I'm pointing out that the presence of Sanders, Fiorina, Carson, Cruz, and Trump throughout this process is proof that the two parties have less control than that which you ascribe to them.