Maybe there are some efficiencies to be found, but we are talking about experimental treatments where the efficacy takes weeks to years to measure. You can't run an overnight benchmarking test.

But in saying that, if a trial is showing very positive results then it can be fast tracked, as it is unethical to withhold that kind of treatment. Conversely, plenty of trials that look good in phase I/II fail in phase III and are terminated early.

To your point, as soon as enough evidence is obtained in Phase III that the drug is better than standard of care, the trial ends early and patients in the trial get switched to it. At least, that was the case in the case studies we looked at in my lectures.