The answer is, the therapy does not improve much. It was controversial when it was approved, because the Phase III clinical trial failed to show statistically significant improvement- lots of people in the FDA advocated against approving the drug (even without knowledge of these rare fatal side effects) but were overruled by Peter Marks, head of the the biologics for the FDA under Biden.

It seems to me to be similar to the approval of the three Alzheimer's drugs which don't really show improvement either- it seems like over the past decade the FDA has wanted to approve drugs that might work for diseases where there was no treatment at all (while saying things "delivering hope"). And it's not gone well, and has not been a good idea.