And now imagine typical end-game set of challenges after last save point in the game:
8-5 - A very long and boring autoscrolling level involving RNG geretated Bob-Omb bombardments.
8-6 - A brutal rising lava section where one slip or dick move from enemy means instant death.
8-7 - A mad dash to gates of Bowser`s Castle with frame-perfectly strict time limit (think Mad Dash from e-world series). A loss of momentum means invariable death by timeout.
8-8 - And epic marathon castle that strongly deserves to have more than 1 checkpoint yet limited by 1. Culminating this is an extremely difficult multi-stage Bowser fight with lots of RNG-dependant stuff, preceeded with long unskippable cutscene.
And you can`t farm lives at all. World 8 has no 1-ups at all and 8-1 is a point of no return.