A failure to do so (if there is no genuine adversity between the parties in practice because they collude on the result) renders the lawsuit dead on the spot (because the federal court cannot constitutionally exercise jurisdiction over the parties, so there can be no decision on the merits) and exposes the lawyers and parties to punishment and repercussions in case they tried to conceal this from/directly lie to the judge, both because lying to a judicial officer in signed affidavits is a disbarrable offense and because this would waste the court’s already undersupplied resources.
Federal lawsuits must satisfy the case or controversy requirement of Article 3 of the Constitution.
A failure to do so (if there is no genuine adversity between the parties in practice because they collude on the result) renders the lawsuit dead on the spot (because the federal court cannot constitutionally exercise jurisdiction over the parties, so there can be no decision on the merits) and exposes the lawyers and parties to punishment and repercussions in case they tried to conceal this from/directly lie to the judge, both because lying to a judicial officer in signed affidavits is a disbarrable offense and because this would waste the court’s already undersupplied resources.