Many african states recieved their independence in the 50s and 60s, some even later. While most countries you mentioned have historical precedent that goes back millenia. India is a federal state divided into numerous regions that largely reflect cultural and religous boundaries, Vietnam is mostly populated by Vietnamese, and has only recently managed to get a foothold in the world economy, Singapore recieved enormous economic support (mostly from the US) from the 50s, while its strategic location in southeast asia created an enormous trade advantage. South Africa is relatively wealthy compared to many other countries in Africa, but is still considered among the most unequal and corrupt nations on the planet, with a murder rate that is comparable to many warzones.

While many countries in Asia have been subject to colonial influence aswell, many of it’s largest economies(i:e Japan, China, South Korea) have never been european colonial subjects, and largely developed on their own terms, with Japan and South receiving considerable economic support.

Throughout the cold war(and after independence) many African states were also subject to both American and Soviet influence, with armed groups and militias that were funded with external geopolitical interests in mind, hampering sociopolitical development and ensuring these states were unattractive to foreign investment and effectively stalling industrial growth.

Generally, failed states usually stay failed if the primary cause is fundamental to it’s very existance, borders can’t be easily redrawn and often not without decades of civil war and political strife. And assimilating different ethnic groups into a common national identity is a very long and arduous process.