I don't know the details of the nexium approval but for novel drugs you don't know it's effective until you test it
The biggest driver of the high cost of drug development are failed late stage (phase 2 and 3) studies. Things look effective in the lab, then look effective in animals, then look potentially effective in exploratory human studies, then when you test rigorously for effectiveness in humans, they often fail. And these late stage human studies are the most expensive
For generic drugs where you "know" it is safe and effective, there's an expedited pathway with fewer / smaller studies. And for slight tweaks to approved drugs there is another process that's more burdensome than the generics approval process but not as bad as a full new chemical entity path
Because doing controlled trials is how you come about that knowledge. Otherwise you're just guessing. You can guess something is safe, but until you try it for real in a systematic way, that's all it is, a guess. That's not knowledge.
Why not? AstroZeneca knew the safety and efficacy of Nexium without doing any research, but they were still required by law to run a couple studies.