If the President doesn't like a Supreme Court ruling, he doesn't have the authority to call all nine justices into the Oval Office to defend it. Any President foolish enough to try would get a lesson in the definition of "three equal branches of government."
And, just as the Supreme Court isn't at the President's beck and call, the President is not at Congress's beck and call. I really don't know where Congress got the idea that it can demand explanations from other "equal" branches of government. Yes, I'm saying executive privilege isn't necessarily a bad thing, regardless of Nixon's attempts to stretch the concept to cover criminal activity.
Of course, because of the Separation of Powers, the President is required to sell ideas to Congress in order to get them approved (think 95-0 vote against the Kyoto treaty even though President Clinton wanted it ratified), the President will often voluntarily talk to Congressional committees. But Pelosi can't demand Bush appear before the House any more than Bush can demand that Pelosi hand him a list of her biggest political supporters. That's the definition of "equal."