Some of the SNES cartridges had an extra co-processor (like StarFox) that your standard Amiga couldn't match
Even without extra processors, the Amiga's chipset was no match to the SNES: look at Street Fighter 2,..
SNES had scaling, rotation, transparency, 4 background layers with independant scrolling, 128 (8/16 color) sprites (32 max on scanline), mode 7 with matrix perspective...
Of course, when optimizing, the Amiga could output nice things, but that most of the time looked more like technical demos than real games since the gameplay was limited because of these tricks (eg. Beast,...).