Section 3 of the 14th Amendment was added to the Constitution after the Civil War, where the very people who committed treason against the Union ran for office.

Section 3 of the 14th Amendment was added to the Constitution after the Civil War, where the very people who committed treason against the Union ran for office. Hence the need to clarify that those people are ineligible. Same thing happened in Germany after WWII, where Nazis tried to run for office and the law was changed to forbid them from holding political office. Virtually all democracies have provision for disqualifying people who are anti-democratic.