When it is the first use in humans, and the main point of Phase 1 is safety, why is there not an exponential turn on of the trial to avoid widespread harm? For

When it is the first use in humans, and the main point of Phase 1 is safety, why is there not an exponential turn on of the trial to avoid widespread harm? For example Patient 1 gets medicine for a week (or whatever), then patients 2 & 3 start their trial, etc. Where is the economic analysis of length of test vs risk of harm by various turn on strategies?