Clothing is a commodity item, even if it takes a week or a month to ship to the US, it does not matter. If you need it faster, raise the prices (or take a hit on the massive retail markup) and ship it faster (air vs. boat).
Having also built a successful print on demand business, we did all our printing in the US because the customers were in the US and the shipping times would have been too great because of the on-demand nature.
The garments themselves come from other countries. Notably, the people creating the designs and buying the FB ads, which target US consumers, are in SE Asia (mostly in Vietnam). Only people in the US will pay $25+ for a t-shirt that costs $1 to $4 to produce.