Uh, we don't really know how our drugs work (especially the older ones). We have a vague understanding of their mechanisms, but we really don't know how they work. We don't even have a clear idea of what the structures of most drugs look like, and how they interact with their binding sites.
Luckily, we don't actually have to know how they work, to know that they work. Instead we use clinical trials and real world evidence to support their use.
(Fun fact: there's actually a branch of drug development called phenotypic drug discovery which actually does away with the understanding of the mechanisms altogether. )
Pharma companies spend a majority of their time trying to make new unique drugs, they just fail most of the time. The ones that succeed tend to be ones that are similar to ones that succeeded in the last, which is why you get multiple drugs in the same class, but it's not all they do. For example, we've essentially cured some types of cystic fibrosis, and there's an effective vaccine for malaria now - all developed in the last 10 years.
I don't want to pretend that the big pharma companies aren't evil, but they do have incentives that align with improving human health.