Because there’s no portable way for a variable argument functions like scanf and printf to know the types of the variable arguments, not even how many arguments are passed.
See C FAQ: How can I discover how many arguments a function was actually called with?
This is the reason there must be at least one fixed argument to determine the number, and maybe the types, of the variable arguments. And this argument (the standard calls it parmN, see C11(ISO/IEC 9899:201x) ยง7.16 Variable arguments ) plays this special role, and will be passed to the macro va_start. In another word, you can’t have a function with a prototype like this in standard C:
void foo(...);