Adding fuel and engines to turn the last stage into a lander adds a lot of weight that has knock-on effects resulting in the launch vehicle needing to be much, much bigger for a given payload. Combined parachute and descent thrusters might be light enough, but I don't think that's ever been done for a human landing. Plus, if a chute fails but you still have fuel, the landing goes from being potentially rough to potentially explosive, which is much worse from a PR perspective for a crewed lander.