[Question] Why are clinical trials so expensive?

While I know that clinical trials such as the trials for vaccines cost hundreds of millions of dollar, I have a hard time understanding why they are so expensive. I read about clinical trials for a vaccine costing 10,000/​study participant. Given that the trials are so expensive they seem to be the bottleneck for medical progress. If we find a way to run them cheaper that would massively accelerate medical progress.

What resources do a good job at documenting the costs of clinical trials and how those high costs come about?