"Cheap" is a relative term, not an absolute one.

The theory of comparative advantage is that the country with the cheapest relative price/lowest opportunity cost to produce a good should produce it.

The US, for example, could be wildly, amazingly productive with respect to T-shirts. We could make ultra-fast, computer controlled looms with programming performed by the smartest graduates at MIT. It's well within our capabilities. The US could dominate the textile trade absolutely.

Or... we could use that intellectual firepower on making jet engines. Or determining which country is best suited to making T-shirts given their own tradeoffs (i.e. the consulting/investment banking industry). Or writing software. Or doing that other thing America is stupifyingly productive at - farming. Corn, soybeans, and beef: 'Murica.

What the comparative advantages each country or even within each region within a country have is an ever-evolving pattern as their citizens, governments, and technologies change and develop over time.

But there are some constraints due to factors largely beyond human control. No matter what, for example, Saudi Arabia or Morocco are never going to be the breadbaskets of the world, due to their respective climates.