Clinical Trials - why the expense?

I've been wondering, why is it so expensive for a clinical trial? Don't they just advertise at hospitals and clinics for people, pay them x amount for a few visitations and then simply submit the data to the relevant authorities for review?
Take a look at the below chart for a breakdown of a couple of aliments. It's expensive, no wonder PAR would find it advantageous to partner with a larger Phrama Company to assist with this. As far as I can understand we are in a good position, our PAR has some $10 M in cash so if needs be we can go it alone, but certainly an advantage to find an appropriate partner for this, and future steps, to bring about commercialisation.
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Source: https://www.centerpointclinicalservices.com/blog-posts/driving-drive-drug-innovation-and-market-access-part-1-clinical-trial-cost-breakdown/
Do your own research of course.