That would mean each person gets a custom pill, right?

Also I'm not a psychologist but from a control theory perspective, there might not be an optimal dose? The optimum might be an interval of doses, and/or the optimum might change with time. And then the consequences of using a suboptimal, preset dose might be at least a good solution. We could probably quantify this mathematically...but stepping away from a control theory perspective: I live this experience so at least in my case, the preset dosages have worked fine.

This exists and is actually the way pharmacy used to work.

Long story short, economies of scale led to today's system.

In the field of pharmacy, compounding (performed in compounding pharmacies) is preparation of custom medications to fit unique needs of patients that cannot be met with mass-produced formulations. This may be done, for example, to provide medication in a form easier for a given patient to ingest (e.g., liquid vs. tablet), or to avoid a non-active ingredient a patient is allergic to, or to provide an exact dose that isn't otherwise available.

https://en.m.wikipedia.org/wiki/Compounding

We can’t be trusted with such responsibility for our own health. Every single one of us would immediately overdose.

~/s~

I would settle for it being seen as okay to vary your own dose. I'm on quite a high dose, and very much need it. But there are some situations (on holiday and just having a lazy poolside day) where I could take less. But despite being up with various doctors it doesn't seem like an option to them.

Fortunately, I've ended up with some different doses because of the meds shortage (getting 2 packs of 20mg instead of 1 pack of 40mg) which let's me do it myself, but seems crazy how little flexibility there is for a dose level that was entirely decided by me trying them and saying which one worked best.