Infix has nothing to do with it. - refers to the subtraction function regardless of whether it is placed in prefix or infix position. Infix will trigger a type error, not a syntactic error — it will tell you that e.g. it cannot call a fixnum. Lisp quite ordinarily requires function names to be separated from their arguments by spaces, which is just a special case of the rule that atoms need to be separated by spaces. That’s all.
Thus + serves more than one purpose: function or variable and both may be different values.
Thus the position actually matters.
What we have below the Lisp syntax are s-expressions: nested lists of symbols and other objects. In s-expressions, symbols need to be separated by whitespace, parentheses or possibly by some special character type (-> terminating character). Characters like +, -, /, *, _, ... are valid characters for any symbols.
Thus Lisp does not require functions to be separated by characters but symbols. That's a part of the reader mechanism for reading s-expressions (-> symbolic expression).
You can't have infix minus and kebab case without something to differentiate between the two.
Lisp chooses to remove infix minus. Other than old-school BASIC, I don't remember anything else which uses tokens for variables.