Some airline systems decided to automate this re-validation, but most systems (including Amadeus and Sabre I think) decided that the logic to do this wasn’t worth it, and that with ET, re-issuing rather than re-validating is a simpler approach.
As far as I know, IATA is behind the drive to reissue rather than revalidate. Amadeus is implementing a product for airlines that will automatically revalidate the e-ticket in case of a change to the booked itinerary - it is already in place for carriers using Altea Inventory for schedule changes.
