The AFCON is not solely to showcase players, though that is a major side bonus.
As I mentioned, having a senior national side compete every two years gives possibilities for countries to attempt to get in and a more frequent rate, especially considering the vast number of countries in Africa. If there were only 10 countries, I'm sure it would be more feasible to have it every 4 years as they're all guaranteed to make it. Right now, only 16 teams can qualify out of 54 total nations.
Additionally, the football infrastracture in Africa is not in par with, say Europe. An u23 tournament would be extra hassle, without the prestige. People remember the winners of the European Cup, AFCON, and COPA over the past few tournaments, how many remember the u23 winners?
I think eventually, they will shift towards a tournament every 4 years, but probably when the infrastracture is there to enable more than 16 teams to qualify , and host nations that can manage that. In the meantime, European clubs will have to make due with their African players missing 3-4 matches every two years (and only if their nation qualifies).
It is interesting that the AFCON is now an issue for us, simple because of Mane. We're in a thread discussing the virtues of an AFCON every two, versus four, years or even if it's shite, simply because Mane may miss a few games? It seems the world really does revolve around us in Europe.