PHP keeps evolving. PHP itself uses semvar, so changing from PHP 7 to PHP 8 implies breaking changes. And PHP has a limited lifetime for security issue support.
Our code does not have to use newer language features. But, the PHP folks have changed things so that existing code can throw errors in newer PHP versions.
WP has already handled most of the problems with PHP 8.0, although some still pop up. They are working on 8.1 and 8.2.
We need to backport their fixes, making sure to get them all.
Here is a list of existing PRs with the PHP 8 label. Each needs review and testing.