Many of the major “neutral” countries weren’t really, but the big thing is that WWI and WWII (at least in Europe) were not primarily wars of territorial acquisition (there was a bit early on in WWII, but that was mostly viewed as territorial reconquest), so there’s no need to open another front just for the sake of it.
This is best exemplified with how the USA got involved in WWII, primarily having stayed out of the war (though not technically neutral) until Japanese territorial acquisition attempts led to the attack on Pearl Harbor. Suddenly that front became active for them but also the USA was able to aid officially in the other fronts.
As for Switzerland, which seems small enough to not be a big concern, it’s so geographically defended (with man-made measures to reinforce that defense like plenty of AA turrets) that beyond at least IRBMs, any attempts to exert military force against them is useless
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