Because it's a two-part ting. Being popular enough and finding a maintainer (from them limited pool of official maintainers there are) who does it.
The AUR is unofficial and no one cares much if updates are delayed there but the offcial repositiories have higher standards and -and that's related to the list of browsers also not in the official repos- maintaining google-based browsers is a pain in the ass (in fact I'm used to seeing warnings because librewolf is once again flagged aus out-of-date but the AUR maintainer didn't have the time to fix the latest build yet).