It used to be a legal requirement for American cable providers to offer them, but the FCC dropped CableCARD in 2020 considering streaming apps to be a reasonable equivalent (https://www.techhive.com/article/578820/cable-box-competitio...)

Comcast still offers CableCARDs to new customers at least (https://www.xfinity.com/support/articles/about-cablecards)