Because it’s so wonderfully effective for the majority, there ends up being a ton of social and economic pressure for anyone even slightly overweight to take it, despite said 10% risk of death.
(Edited to add):
“The Pill,” by Meg Elison
https://escapepod.org/2022/11/03/escape-pod-861-the-pill-par...
https://escapepod.org/2022/11/10/escape-pod-862-the-pill-par...