It was popular during a period when folks used science to make policy. It was also popular as a thought experiment on how to improve humanity and society as a whole but when faced with the reality that eugenics required the survivors to ignore the screams of the oppressed, ethics won out.

Don’t forget psychology. Turns out a lot of its like 90% of it is founded on a mix of religious dogma and eugenics. Remember that infamous Canadia Nazi Jordan Peterson who nearly killed himself with a paleo diet and then went on a ketamine bender in Russia? Psychologist.

If you want people to be less stupid or gullable, the answer is founding education programs to teach them scepticism.

About the irrationality, I think it’s part of what makes us human, biases can be overcome with some knowledge and humility but some things about humans like doing something for no reason or just because it fells good will never dissapear.

As for just trying to create healthy humans, we can see that idea failing with crops. We selectively bred the healthiest crops, then everyone wanted that best crop, and now the pests for that crop flourished, meaning they’re usually even more sickly than other crops and we have to constantly keep them alive with pesticides.

Eugenics in general has many problems, though. The most fundamental problem is that, whatever one might view as an ideal human, is completely arbitrary.

Even if we ignore obviously existing personal biases, there is no fundamental reason why a muscular man is good, for example. In the next few decades, we might see full automation of manual labor. Then those muscles are irrelevant. And we might see ever hotter temperatures. Then having a bulky body might actively be bad for cooling off.
Or maybe tomorrow, there’s a massive volcano eruption, which causes oxygen levels to fall and food scarcity. Then a slender man might be best adapted to that situation.

There’s just a million ways in which our situation can change all the time. And the best strategy for dealing with that is diversity.

Which is the second fundamental problem with eugenics, it necessarily reduces diversity.

Diversity is also what prevents singular pests/illnesses from being able to wipe us all.

But ultimately, diversity is also great, because we live in a society. People with different strengths and weaknesses can work together, usually indirectly by just taking up different jobs and paying each other to perform our respective jobs.
In particular, a weakness in one field can also push us to develop greater skills in other areas. Had I been able to become a super model, I wouldn’t have pursued an education as fiercely, for example.

We’ve already collected DNA samples from people around the world to map the genome, it’s how we even know what genes may cause disorders in the first place. This did not require us to create some genetically ‘pure’ hypothetical gene sequence for the ideal human. What you’re suggesting is still eugenics and still bad for all the same reasons.

There may be such a thing as an “average” human genome though I doubt such an amalgamation would be able to produce a viable or even desirable organism. There is absolutely no such thing as a “clean” human genome, and attempting to create one would be a damning demonstration of a deep misunderstanding of both evolution and biology in general.