This is why the RetroPie download page only has ISOs available for models as recent as the Raspberry Pi 4. However, as "abj" alluded to, that's not the only way to install RetroPie on a Raspberry Pi.

However, if you've gone looking for a Raspberry Pi 5-ready RetroPie ISO image to install the operating system from since the Pi 5's October 2023 release, you've probably come up empty.