why can’t we use any modifiers in float data type in c

why can’t we use any modifiers in float data type?

#include <stdio.h> 
int main(void) 
{ 
    signed float a; 
    short float b; 
    return (0); 
} 

If tries to use it ,the compiler gives compile time error.

error: both ‘signed’ and ‘float’ in declaration specifiers
signed float a; 

error: both ‘short’ and ‘float’ in declaration specifiers
short float b;

Why does compiler show this error?