The boundaries of colonies were drawn without regard for Africans by the European powers during colonization and often grouped together people of rival ethnic groups, which led to ethnic strife in the post-colonial period. Considering that the modern nation-state is generally organized around ethnic groups (people with a common language, history, values etc.), why didn't the leaders of the independence/nationalist movements organize their countries around [similar and/or friendly] ethnic or linguistic groups, and redraw the political boundaries of Africa when they gained independence?
Please, no more half-assed answers. Remember the rules. As was explained in the excellent recent meta post on "What it means to post a good answer in /r/AskHistorians": If you're choosing to answer a question in /r/AskHistorians, there are three questions you should ask yourself first in turn:
Do I, personally, actually know a lot about the subject at hand?
Am I essentially certain that what I know about it is true?
Am I prepared to go into real detail about this?
The process of 'decolonization' is at the heart of this question, but so is the nature of the colonial state. Colonial administrations were, in essence, pro-consular states. In other words, they reproduced all the apparatus of an independent state -- infrastructure, executive, judiciary -- with power vested in the colonial governor as a proconsular officer with effective sovereign authority. In addition, the colonial administration exerted authority partly by a divide-and-rule strategy: they promoted 'tribal' difference and (through indirect rule policies) the power of chiefs, each with their own population and with rivalry towards each other.
In essence, in order for political decolonization to occur, anti-colonial forces had to capture state and had to overcome this divide-and-rule strategy. This meant that the most effective anti-colonial actors (1) were formally educated and had spent time themselves working for the colonial administration or at least understood it well and (2) promoted a 'nationalist' sentiment to cross-cut ethnic difference. Of course, in some colonies (notably Nigeria) many nationalist political parties were in fact ethnically based. It's no surprise that the result was the Biafra war -- which threatened to in fact sunder Nigeria into ethnically-bounded states. In most others, however, the nationalists managed to at least for a time bring together the various ethnic groups under a 'national' anti-colonial banner.