There are many causes for state weakness, there are many causes for state strength. State legitimacy, corruption, homogeneous/heterogeneous societies, unstable governments, uneducated workforce have all been named as the reason why an economy of the country is not working, but there is never one single reason why something does not work There is not also always a continual growth, and some countries can grow much bigger and stronger in some years, only to be hit by a crisis and then deal with a recession.
Saying that countries have not been created in the most natural way, does create frictions, “The state structures and boundaries created by colonial powers and post-colonial leaders bore little resemblance to those that preceded colonialism“, and yet Belgium could be described the same way, and it manages to not only survive but thrive(while having internal discussions between Flanders and Walloon). On the other hand Ethiopia, Congo, Madagascar have been kingdoms before the colonizers took over, and the countries created after the fall of colonialism have been similar in shape and ethnicity though not developing.
Corruption is one form or other we can find in many developed countries in the world and there does not have to be such a problem in a particular African country, for example Botswana which holds 32nd place on the Corruption Perception Index (ahead of Malta, Slovenia, Israel,...) and yet still is not doing as well as that index would say, because more than 30% of the population lives below poverty line.
Heterogeneous countries, countries where the population is not in the most part of the same nationality or does not speak the same language, have been doing extremely well, like many ex-colonies, like USA, Canada, Australia, or countries like Spain and Switzerland, and the homogeneous like Sweden and Finland, and yet it did not work as well for African states regardless them being homogeneous, like Somalia(common