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.

If that sounds similar to the Raspberry Pi ... can be more powerful than a Pi and also be found for a much lower price. He’s using one of these to run RetroPie, which actually turned out ...