That's why you have anti-dumping laws in the western world.
I haven’t seen anything “Made in Africa” in North America.
What costs less for you in America may not be the same as what costs less in Africa, because of transport costs.
My guess is that these countries have protectionist duties, which just makes goods more expensive for locals. Textiles aren't a very strategic industry. (Yes, everyone needs them, but it's not hard to find a selling country).
I'm just not sure of relevance of the cost of asian versus african shirts in the U.S. It's not just shipping containers, but the rest of the transport network, how many trucks are available, the retail networks, and the middle-men to facilitate the trade, the different labor and cost of living, currency exchange rates, regulatory stability, etc.
Seems like there are a lot of confounding variables and it would be difficult to draw on what t-shirts currently cost in the U.S.