I agree with you, one cannot just look at numbers of countries on an continent or number of population. Of course Asia has much more inhabitants, but the two largest countries India and China are practically not present in the football World. African teams are very passionate, but so far haven't had much success in world cups. You could give them more slots, which of course would increase the chance that a few teams could come further into the tournament, but it wouldn't raise the quality of the tournament probably.
If we look at the performance of Asian teams against European teams, it is easy to understand that Asian teams are very weak. They are far behind. While teams like Korea and Japan have been able to improve themselves somewhat, they are still much weaker than European or American teams. The European leagues are very advanced. We don't see any better league in Asia than that. More time is needed to improve football in Asia.