If you wrote a version of Ruby that was still infix but where you needed to put spaces around all symbols, then "foo-bar" would work fine there, too.
But what’s also at stake is whether that language would be readable to humans without mistakes. Part of what makes Lisp work, IMO, is that you never try to type foo - bar, so you never run into an accident by mistakenly typing foo-bar and having the parser think you meant a hitherto undefined symbol.
Whether that’s a worthy tradeoff is a matter of opinion, and I’ll take Avdi’s word for it that he believes the benefits of calling symbols foo-bar outweigh the odd time you might type the wrong thing. I’m just pointing out in addition to the “whitespace is the separator” argument you both make, there’s also the “foo - bar doesn’t mean foo subtract bar” argument.
Unless you have a function called "foo", and you want to pass it the subtraction function as it's first parameter and the value "bar" as its second. In which case (foo - bar) is perfectly valid, reasonable, and completely different from (foo-bar).
Even though Lisp is not an infix language, foo-bar does mean something different to the parser than foo - bar because of whitespace.
I’m sure you knew perfectly well that I meant you never type foo - bar meaning foo subtract bar :-)