Switzerland was not a threat to Germany (it remained neutral for centuries), it has few natural resources, it was not in the way (you can go around it), and capturing it would be really hard (population is armed, trained, and motivated, and mountains perfect for defense)
In addition, Germany was buying Swiss optics for its weapons (and so did Allies), all the Nazi leadership had money stashed in Swiss banks, they were a useful as a diplomatic channel and spy waypoint.
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