Comment by estearum

The alternative is "have a complete mechanistic understanding of biology." People are definitely working on it, but we are nowhere close to as complete an understanding as we need. The evidence for precisely this claim is in the failure rate of clinical trials, into which we encode our best-known understanding of biology and we still end up being wrong 90% of the time. And that's not including all the pre-clinical candidates, which drive our wrongness rate (i.e. knowledge gap) to 99.99999%+