I totally understand where this question arises from. Until we find another alternative, the only two options left are humans and animals. The objective of pre-clinical study is to find a solution i.e, a drug or a new device for humans (in general). There is also certain aspects of ethics during pre-clinical trials such as GLP without which it is tough get an FDA approval. Furthermore, from a company’s perspective, it’s much more economical to do an animal study first and prove the drug or vaccine or a new device is working well rather than spending millions of dollars on human subjects.