Why? When the latter is just as effective but also much quicker?

It isn't just as effective for me. No matter how much I'd like to, I can't review code with the same thoughtfulness and thoroughness that I apply when I write it. I know the same is true for the people who have reviewed my PRs as well, but maybe its different for others. I do use Copilot but mostly it only generates one liners for me that save a little time.

Empirical studies on large-scale projects employing informal code review (the study I'm talking about monitored the Qt project repositories) suggest that humans have a very low impact on error rates. Reviewing more than ~200LOC every couple of hours makes the effect disappear.

So you're not alone. You can even point to the plethora of "find the undefined behaviour," tests: humans are really bad at finding errors in code.