Because you didn't listen to me. You have them in melonds-0.8.3 > switch > melonds, I told you to move them to just switch > melonds. It's not going to work unless you do.
There's the root of your sd card and there's a switch folder on the root, inside that is melonds, put them in there.
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It's on the first post of the thread about MelonDS on gbatemp.
This is my port of the Nintendo DS emulator melonDS for the Nintendo Switch. To use the emulator, place your legally dumped Nintendo DS BIOS files in the sdmc:/switch/melonds directory with the names "bios7.bin", bios9.bin", and "firmware.bin".