It's a very good question:

"But Recovery’s incredible success should prompt us to ask a more challenging question: why don’t we do this more often? The question of which drugs to use was far from the only unknown we had to navigate in the early days of the pandemic. Consider the decision to delay second doses of the vaccine, when to close schools, or the right regime for Covid testing. In each case, the UK took a calculated risk and hoped for the best. But as the Royal Statistical Society pointed out at the time, it would have been cheap and quick to undertake trials so we could know for sure what the right choice was, and then double down on it."

https://www.theguardian.com/books/2023/mar/06/the-big-idea-should-governments-run-more-experiments