What makes it difficult is the word “prove”
It turns out it’s obscenely hard to make a drug that’s good, and even harder to prove that it’s good.
So how do you do that ethically? How do you justify taking off something that you know works to some extent and try something completely new or worse placebo? ie don't you have to construct the trial in the context of existing treatments etc?
These are the kind of challenges that makes drug development slow - in the end you don't do one trial, but a series of trials, slowly building confidence and making the case.
Often that's what takes the time during the clinical phase.
Of course it would be much faster to go straight to a big trial that would show how well your treatment works in conditions optimal to it - however that kind of 'move-fast break-things' approach involves potentially breaking things which happen to be people.
Regulation just reflects the cautious 'first do no harm' philosophy.
Now let's be honest - big pharma will simultaneous complain about regulation and the cost of development, and at the same time know it creates barriers to entry - there is always some frustration about the slowest of regulatory authorities to adopt new methods - however you wouldn't want your regulatory to be gungho.
Just to be clear, most drug trials for anything where we have an effective treatment are not “new drug vs placebo”, but instead “new drug vs standard of care”. Thus the goal being to prove it’s better than what already exists.
If the standard of care is already good and you don't need a placebo - then you have another problem - you probably are going to have to do quite a big trial to get the stats to show a significant difference, and you are going to find it harder to persuade people to participate with an experimental treatment if there already is a fairly good treatment.
The whole point about the challenges with clinical trials is that it's not an intellectual exercise in designing the perfect experiment and 'just doing it'.
It's about persuading yourself, the regulators, the doctors and ultimately the patients that it's something you should try - and before you've done your first trial you don't have any human data to show it's safe and effective - all a bit chicken and egg - the solution is often to move slowly in stages.
We can't do any of that for drugs that affect emotions and consciousness because we're barely in the stone age of our understanding of the brain and the technology we have to affect it.