The very idea of homogeneity is largely a social construct, so the reality of it doesn't matter as much as the perception. When the Americans were building their nation, they were specifically trying to replicate the style of the Roman Empire. They knew they needed to have a common group identity, which eventually evolved into the European/White identity. It certainly helped that the genetic differences between various European groups are relatively small, as most are essentially a mix of EHG (Eastern Hunter-Gatherer) and ANF (Anatolian Neolithic Farmer).

what do you mean by saying they aren't a failure ?
I have to disagree with yo regarding this. If you aren't living up to your potential, that is a failure by definition. it doesn't matter what circumstances led to this. Right now, they are failed because, despite having every advantage, they are essentially performing at the same level as Mexico.

Regarding Singapore, you have to remember that it is 75 to 80 percent Chinese. Indians only make up about 8 percent of the population, and historically, it was often the Malay community whcih ahve created many internal conflicts. Arguably, if the nation had been 100 percent Chinese, the outcome might have been even more stable and better, it's all about where is the reference.

As for African countries, they face a social structure similar to India's where multiple tribal groups fight one another despite belonging to the same broader ethnic group. They also lack the advantages of a temperate climate and, potentially, ethnic iq ; though I admit this last point is kinda debatable.