It’s a matter of Taiwanese law.

“Since Taiwan has related regulations to protect its own technologies, TSMC cannot produce 2nm chips overseas currently,” Kuo said at a meeting of the Economics Committee in Taipei, reports Taipei Times. “Although TSMC plans to make 2nm chips [abroad] in the future, its core technology will stay in Taiwan.”

https://www.tomshardware.com/tech-industry/tsmc-cannot-produce-2nm-chips-overseas-until-domestic-output-becomes-more-advanced-confirms-taiwanese-govt-official