That's not the way it works. Concessions can and have been made to accommodate the president's schedule. This case in particular is an uncomplicated dispute that should not involve the chief executive in drawn-out depositions or much time in court. It's a simple question of believing her or believing him, not an antitrust matter that could go on for years. Conversely, establishing a precedent protecting all presidents from lawsuits while in office would be unfair to plaintiffs.
Memories fail over time.
Evidence gets lost or destroyed. In civil actions involving divorce and child custody, for example, lives are on hold and children grow and change if court orders are postponed for four or eight years. If Mrs. Jones has to wait until the next century for her day in court, justice, for her, will have been postponed to the point of denial. The president deserves respect and cooperation on scheduling from the courts.