Why do only US makers focus on time coherency?

Bombaywalla is incorrect-JmLabs speakers are time AND phase correct. They design the structure of their speakers and their crossovers with that in mind. Don't forget that it all depends on listing distance and height of the listener relative to the loudspeaker drivers(especially the tweeter) in order to benefit from such a design.
Wilson Audio's are also correct, and are set up when you purchase them for your arrangement, but then again, they're American also.