A big issue is the speed at which teams are expected to deliver. If every sprint is expected to deliver value to the user, there is isn’t enough slack in the system to go back and prune the code to remove cruft. People end up cutting corners to meet deadlines set by management. The corners that get cut are the things that are invisible in the demo. Security, documentation, and all the chewing gum holding it all together.
And once a level of "story points" is achieved within a Sprint you can't go backwards and you can't deliver less value to the Customer. There is no room for re-evaluation. Forwards, moar!
As per Tame Impala's Elephant:
He pulled the mirrors off his Cadillac
Because he doesn't like it looking like he looks back
Looking back gives the impression of missteps or regret. We have no such thing!
