Arnov Sharma has put together a beautiful all-in-one Raspberry Pi box that's portable and made for gaming on the go.

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.

Inside you’ve got the aforementioned Raspberry Pi 4 (with a Noctua fan to keep it cool), an external hard drive, a HDMI to VGA converter with scanline generator to drive the 4:3 ratio Eizo Flex ...