You probably not search correctly. They use general pcb manufacturers because from a pcb standpoint, keyboards aren't any special. They are actually rather easy to make (unless you have any weird specific features).

OSHPark (based in the US) is probably the most well known PCB manufacturer for the general public outside of China. Most people would say they have superior quality but you pay more, obviously. The typical tradeoff.

PS: I'm sorry. I just noticed you asked for the entire keyboard manufacturing pipeline rather than PCBs only... I didn't delete this post because it might still interesting for OP...