Because it’s unbelievably hard to 1. Produce a molecule that does what you want it to in the body 2. Figure out how to get that molecule where it needs to be

Because it’s unbelievably hard to 1. Produce a molecule that does what you want it to in the body 2. Figure out how to get that molecule where it needs to be in the body 3. Prevent it from doing all sorts of other gnarly things 4. Convince people you’ve succeeded in doing 1, 2, and 3 to the degree they’re willing to volunteer themselves to put this new molecule inside their bodies — without incentivizing them in any way 5. Convince enough people to do that and collect enough data about them to know for a fact that your drug does X and does not do Y 6. Convince regulators that not only have you done 1, 2, 3, 4, and 5, but that it’s also better on some dimension than the currently available treatment In short: because it’s ridiculously hard. If it’s easy you should go give it a shot, help millions or billions of suffering people, and make billions of dollars in the process. Why wouldn’t you?