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.

The Nazis did that, breeding blonde-haired blue-eyed people (mostly combining German men with I think Swedish women). Most of them were murdered after the war ended (despite only being innocent children), because people thought that 1. Nazis have to be mentally different and 2. same for women who “agree” to participate in this program, so they thought that of course the offsprings would “also” be mentally different/disabled.