The 18th, and 20th to 26th amendments, as well as the ERA and the DC Voting rights outdated proposed amendments, included time limits to their ratification. The time limit clauses vary in shape, form and place among the amendments, but that's not the subject here.
What is the rationale behind such time limits included for ratification of constitutional amendments ? Why would Congress at the same time agree and partially withhold a proposed amendment like this ? I'm assuming it has little to nothing to do with the contents and merits of the amendment themselves, because every proposed amendment since 1932 (20th) included such a limit.