> Probably the same reason the Space Shuttle wasn't cancelled even through pretty early on it was clear that it didn't make sense.
Upper management decisions are seldom made for good technical reasons.
> SLS's existence has nothing to do with upper management. This is the child of Congress who funded it without any sort of mission.
Which is a big part of why Artemis is kind of messed up - neither SLS nor Orion was designed with the mission in mind. So Orion has to go to a relatively high near "rectilinear halo orbit" instead of Apollo's "low lunar orbit" because the SLS/Orion system doesn't have enough delta-V to get to the superior orbit and back.
Upper management decisions are seldom made for good technical reasons.