What else would you want to do with them? Maybe in rare cases you'd want to structure them into an array or something, but the inverse isn't possible either [e.g. func f(a, b, c int) -> f(<destructure array into arguments>)] so it isn't like it is inconsistent.

Perhaps what you are really trying to say is that multiple function arguments is insane full stop. You can pass in an array/tuple to the single input just the same. But pretty much every language has settled on them these days – so it would be utterly bizarre to not support them both in and out. We may not have known any better in the C days, but multiple input arguments with only one output argument is plain crazy in a modern language. You can't even write an identity function.