China is huge in area too. You don't have one billion people right across the street. Anything that is across the street is, by definition, a localized thing involving far fewer people, which you can totally have in US+CA+EU too.
not just an ocean, the various governments and visas make it hard to move people you need
Visas are a political human construct subject to change, not a immovable force of nature. The same visa keeping workers out, can always be removed or changed over night if desired to achieve the opposite effect: move masses of skilled people in. See operation paperclip.
Not just people, parts too. Importing parts from the USA to the UK often takes longer and has more paperwork than from China.
Also don't forget tolls / tariffs. Building products that relies on parts from different countries (at least from/to USA) reduces the margins compared to a unified Chinese market.