Git commit messages are written under time pressure, change logs allow you to refactor the history and clarify things as facts become known.

When you want to publish your changelog to your users you probably want to have some standards around your commit messages and when you do, from the commit history you can then automate the creation of a changelog.

https://www.conventionalcommits.org/en/v1.0.0/

Different audiences. A Changelog is read by the users of your software, the commit message by the developers. It's rare that a commit message is suitable for both.

Maybe I’m missing something but git releases are a pain to search through. A changelog is searchable or I can scroll through and figure out where breaking changes or a particular bug was fixed.